I think the point you were replying to is you used to be able to buy a base machine and upgrade it yourself later. The 2012 MBP sitting in the drawer next to me right now, for ex, got drive and ram updates. So did the 2014 mini server I have. The og Intel mini I have around somewhere got *CPU* (core 2 duo) ram and drive updates.
My 2013 Mac Pro (basically the equiv of the studio) has a replaced CPU, drive, and ram too. On the extreme upgrade-ability side I've updated my 2010 5,1 Mac Pro on nearly everything except the backplane (gpu, cpu, ram, nvme boot drive, wifi, bluetooth, usb3 card, and 10gbe network card) in the time I've had it. Even the 2010 iMac allowed replacement CPUs and GPUs, though it was rare that someone actually did.
back on laptops though it was common up until recently for ram and drives to be replaceable, so you could buy the base and upgrade it later. Now you have to get the specs you want for the lifespan of the machine upfront - and some of those upgrade costs are pretty steep.
Now I get why Apple did that, especially with AS and the SoC (though we'll see if that painted them into a corner on the Mac Pro), and I don't disagree with the reasoning (at least on AS) but it is frustrating when buying a machine and budgeting.