The iPhone 5C already has a backdoor; that's the problem. Furthermore, you can be certain that the NSA and other agencies can get in through that back door. Apple winning or losing makes no difference to that. But if Apple wins, it gives the appearance that your data is protected when in fact it is not. Furthermore, if Apple wins, it will give more ammunition to people demanding laws that require explicit backdoors.
So in your opinion we are damned if we do and damned if we don't? The lack of completely secure phones today (or at least back when the iPhone 5C was sold) completely invalidates any potential advances to make them more secure in the future, and if Apple wins in court then the legislature will (despite having failed to do so during the first encryption debate) of course pass laws to grant such a back door in the future. That's an awfully pessimistic view. If legislators from my state start supporting such a bill, they will get an earful from me, and I imagine that most of the tech industry will throw their lobbying weight around as well to prevent such a bill.
We know such a phone would be affordable, easy to use, and popular: there would be no user visible changes. Nor would it be any more expensive, because Apple already has a custom, secure crypto chip that could have implemented the PIN wipe securely without backdoors and at no extra cost.
You may believe that, but I see no reason to believe you are correct. There is a cost, even if not in money, to a completely secure phone. The costs are time and the hassle of remembering, or sharing it when you ask someone else to use your phone for you, etc. Those may be small costs, but to some they are big enough to result in many people still not having even a 4 character pin to protect their phone. You and I may value security (probably to differing degrees), but many don't at all, and requiring a pin that could wipe the phone if a kid gets their hands on it and tries too many wrong passwords/pins scares many. Myself included. I've been locked out of my phone several times because my 2 year old got the phone and tried to get into it, or because it became activated in my pocket some how and ran up several attempts without any deliberate action on my part.
Security may be easy in a technical sense, but to make something that is both secure, easy to use, and desirable to a wide range of buyers all at the same time is something different entirely. Apple prioritized ease of use and desirability over security. Their call, don't buy if you don't want, but they've increased their emphasis on security over time. Maybe they'll never get to where you'd like them to be, but I'm glad for any improvement since I hate the Android and Windows phones I've used thus far. A little protection is better than none.
My guess is that the weak security architecture of the iPhone 5C was deliberate
And unless you've got real evidence, that is just an opinion. And not even one that is logically consistent, since one of the issues at the heart of this case is that the iPhone 6 and 6S are even more secure, meaning the work around that the FBI wants Apple to implement for the 5C won't work on these newer phones.