The pressing question though is whether there's a void for someone else to fill if Google starts making Android a true iOS clone by nixing sideloading. They've inched closer than ever with Lollipop, and I don't see that changing going forward. Meanwhile, no sideloading means that Amazon loses whatever Android customers were using Android phones. While the Fire tablets are surely the biggest slice of the Amazon App Store/Music Store pie, I don't know if they'd just pack up and go home if Google locked them out. Conversely, I don't know if the disappearance of F-Droid, Appbrain, AAS, and other third party app stores would be the tipping point that would prevent people from pining for the Galaxy S7.
Is Android too big to fail? At this point, I'm stuck saying 'yes', at least for now.
It's a difficult question.
Can you side-load on an iPhone? Yes, you can. There are three ways:
(1) Jailbreak the device; some people are willing to do this. The preeminent reason is to work around carrier limitations on tethering/hotspotting to get around the fact that the carrier has unlimited data on some phone plans, but either does not permit, or has capped charges for, tethering/hotspotting through the phone, or through mobile hotspots. Other than a few applications to work around Apple/carrier agreements, or add functionality Apple doesn't/won't approve, this is not so big these days.
(2) Enroll the device as a developer device using a developer certificate. This allows you a limited number of devices, however, and isn't useful for wide scale distribution. It's a dead end.
(3) Enroll the device as an enterprise device. There are already Chinese "app stores" which do this; you enroll voluntarily, or the phone comes already enrolled when you purchase it from the vendor, and they install an enterprise cert, signed by Apple, which allows them to run their own distribution system. Typically, they pirate U.S. Apps, rewrap them, resign them, and then sell them on the cheap, giving no money back to the authors. In addition, there's often malware included with the applications sold this way. Apple hates it, and I have no doubt, there's active work on enterprise support to prevent this, going forward.
So side-loading isn't the biggest issue, since if there's a will, there's a way, and Android would also end up with methods similar to these thre to increase the difficult of, but not eliminate, side-loading.
The flip side of this is that, unless they do something, the pure *volume* of malware for Android will almost certainly kill their viability as an app platform eventually, even if it's not going to happen today or tomorrow.
The biggest problem with Android, with regard to apps, is that there are are too many targets.
Apple is in the process of screwing themselves over this way, for short term monetary gains that will likely not have long term value for them. At a minimum, a developer has to target 7 iOS platforms at this point in time - even if that ends up being wrapped up in a single distribution package, so it looks like a single thing in the App store. In addition, video content is split between 4:3 and 16:9 aspect ratios. Apple eats the transcoding costs and the duplicate CDN costs for these, but the decision to change the device means twice as much content has to be carried around.
But 7 is manageable, if you are targeting the same OS version, or an earlier but forward compatible OS version for all devices.
The Android ecosystem is the wild west, in comparison. And automatic updating of Android versions on older devices won't gloss over hardware differences in input methods, sensors, and so on.
(1) Yes, there's room. If Samsung wanted to own this, they probably could, just by standardizing minimal feature set on their entire product line, and then forcing version updates on the carrier, or making sufficiently compelling new devices that the carrier doesn't think the version updates will impact their 18 month subscriber contractual lock-in model for doing business.
(2) The side-loading thing is not that big an issue. The Android App market isn't that big these days, and it's balkanized enough through device specifications and other capabilities that, everything else being equal, it's possible to create an 800lb Gorilla in the App store space, and have it stick. If Samsung wanted to do this themselves, they surely could do that too.
The problem with Samsung doing this, however, is that their laptop, tablet, and mobile phone divisions are -- in fact -- separate but affiliated companies, all with their own engineering teams, design teams, supplier contracts, and so on. So Samsung would have to change *themselves* to do this.
OK... the Amazon problem...
It's not a problem. Although they've certainly go a more or less portal-play going with their application and content for their Android devices -- all they had to do was default it to pointing at their servers, and most people don't change defaults, as we've seen by the search engine lawsuits against device vendors and against Chrome.
I think that this could be handled by having a master store signing signing certificate, and Google - or come consortium, including Samsung -- controlling the signing certificate. That may end up with, say, 5 "trusted stores", and the members of the consortium would all agree on some minimal feature set on the devices so developers could target a single base model, and they could agree to lockstep their version updates, etc., etc..
Or Google could do it for them, and swallow the bitter medicine on their behalf. Part of that would be the fact that the Google Android App store takes a 30% transaction fee, and they'd have to let the vendors whose certs were signed for them by Google, take all or most of that percentage. And that would lead to competition, which would have to happen in the consortium, for fees. Which could lead to RICO violations, unless they legally structured their price fixing agreement to be an emergent property, rather than an explicit agreement (i.e. it's a "natural" saddle point for the rules under which anyone can agree to operate, rather than an explicit agreement to price fix).
To get back to the original question now...
If all that were done -- and it'd cost Google considerably to do it -- then no, there's not a room for a third party.
If, on the other hand, the question really was...
"Does Microsoft have any hope in hell of breaking into this market at any point in the next 3-5 years, or are they going to be stuck at a 5% market share for Windows Phones?"
Microsoft could break into the market. And they could wield the App Store model that Apple currently wields, rather effectively.
But, as they do not build their own hardware, as Apple does, they'd need to do something similar to what I've described above. And for them, it'd be even more galling. But probably not as galling as it would end up being if either they or Google attaced the problem full wold (long term) be fore the carriers. 8-)