I have an ANCIENT (>10 years old) Dell XPS desktop machine - and last week, the motherboard failed. Went to Fry's paid $65 for a new motherboard and $120 for a new CPU (which included a new cooling fan). My RAM modules were too ancient to run in the new motherboard - so I spent another $60 for a couple of RAM modules. To my surprise, the original power supply, graphics card, hard drive, DVD drive and case all fitted perfectly - and a simple reboot got me back into Ubuntu as if nothing had happened - I was back up and running in an hour.
Sure, the CPU socket had changed - and my decade-old DDR-2 memory wouldn't work in the DDR-3/4 motherboard - but aside from that, modularity worked 100% perfectly. I could have chosen from a dozen different CPU's and a similar number of RAM suppliers and any one of a dozen motherboards - and the outcome would have been the same. None of the replacement parts were made by Dell. The screwholes for the motherboard matched up perfectly, the cutouts in the case for the connectors and graphics card lined up nicely and even the connectors for the buttons and USB ports on the front panel plugged in perfectly. The various blanking pins on the connectors prevented me from plugging in the various wires into the wrong connectors...I could have done it without the instruction books.
So the desktop PC "standard" is already an incredibly modular system. The problem is that (by modern standards) it's physically huge.
For small systems like IOT devices, the cost of "the computer" including graphics, networking, RAM, long-term-storage is down to $10 or less...so modularity at that scale is just pointless - increasing the cost by adding connectors between the parts is just silly.
For systems at the scale of a cellphone, modularity is a tough sell because the physical form-factor has to fit perfectly with the shape of the battery and screen and heat management is a big issue - so making a *usefully* modular phone is challenging.
The real issue is modular laptops. It's a real pain if you screen gets cracked or your motherboard or power supply fails. But you don't need modularity at the electronics level - it's all about modular cases and connectors. You could take pretty much any laptop design and simply declare that to be "THE STANDARD" and manufacturers could come up with replacement electronics, storage, screen and keyboard units.
It doesn't take clever design, it takes the political and commercial agreement of a gazillion manufacturers to pick a form-factor, connectors and other interfaces AND STICK TO IT for a minimum of 15 years.
Simply coming up with a new laptop design and declaring it to be "THE STANDARD" is useless unless you can get a lot of very large companies to sign up to it...and that ain't happening. This isn't a matter of technical innovation - it's all about the politics of those big businesses.
Software has been relatively "modular" for a very long time. You can buy software, download free software or write your own - and it's pretty simple to make it work on the trifecta of OSX, Windows and Linux - and trivially easy if you can make it web-based. But it's very evident that the business model of most companies these days is to lock you in to buying music/video/apps from their "app store"...that's where the $$$'s are...so expect to see more moves like MS's efforts to lock down Win-10 so you have to buy apps through their store.