Apple said entire point of the redesign is to make the machine modular, so that it can be upgraded down the line, specifically addressing this concern. I guess it's up to each person to determine for themselves whether to believe it.
Modular so you can upgrade it only with Apple parts?
Well, if it was like the old pros, modular like: RAM, HD and Graphics cards could be upgraded.
CPUs as well
Have they ever limited upgrades to Apple parts on a machine before?
On pretty much every other product line, yes.
My Mac Mini's RAM says otherwise. Pretty sure that, for the vast majority of its history, the same goes for the iMac. And, obviously, the Mac Pro, even in its current iteration. What other product lines are you thinking of, aside from laptops?
The iMac only allowed RAM changing from what I know and it only had two slots. Changing the hard drive was a surgery that very few people are going to do. Apple is now in the habit of making the RAM and SSD part of the main board in the name of space saving.
OWC (https://www.macsales.com) made it pretty painless to upgrade an iMac's hard drive to an SSD. They sell a lot of kit for various machines. Just saying.