I upgraded both the 2012 Minis I have owned to 16GB the week I got them. The real question is would I have paid Apples ridiculous price to get 16GB BTO if upgrading them myself was not an option (like with the 2014 model)?
For the first one..... no, most likely not, and as a result I would have never tried OSX and become a Mac user. It was to be my main desktop, 4GB was not enough, and Apples upgrade price likely would have put me off the whole experiment.
The second one, I'm not so sure. It's just used for streaming to my TV, so 4GB is enough. I just happened to have 16GB so I popped it in. But not being able to upgrade would limit the Macs future usability should I want to re-purpose it, so reducing it's longer term value to me.
I'm not so much opposed to soldered RAM (although I'd definately prefer it not to be soldered), just the silly price that Apple charges to get a decent amount. If the BTO 16GB option was somewhat similar to the cost of buying 3rd party RAM i'd be fine with doing that. Heck, even if it was 50% more i'd pay it, but when I bought my first Mac it was 4 times the cost of doing it myself. Not cool! Other than RAM I paid for it to be fully maxed out. 2.6i7, fusion drive etc, Apples RAM price would have put it over the top.
Now that Apple has me hooked, I will likely continue to use Macs, but if their current lineup was what was available when I looked at buying my first Mac, I probably would have stuck with Windows. A big part of that is lack of upgradability. It's also the main reason my primary desktop is now an old 2009 Mac pro, and there is nothing in the current lineup that I want to replace it with.