The current state of Macbooks has Apple is moving to disposable units.
In your opinion, what is the best/last year/model for 13" and 15" MacBook Pro machines that can be repaired and upgraded? Can they really be upgraded to compete performance-wise with the newer generation processors and electronics? What is gained or lost with either choice?
The "last upgradeable" MacBooks are the mid-2012 Pros:
13" 2.5GHZ i5 dual, MD101LL/A
13" 2.9GHZ i7 dual, MD102LL/A
15" 2.3GHZ i7 quad, MD103LL/A
15" 2.6GHZ i7 quad, MD104LL/A
15" 2.7GHZ i7 quad, BTO (build to order only)
These are the last Pros to have expandable RAM, although they will only go to 16GB, and they also have replaceable hard drives, so conceivably you can put in a 1TB 2.5" SSD. You can also replace the optical in these machines with a caddy that will hold a second hard drive, in which case you can double your hard drive capacity.
These machines are still reasonably powerful by today's standards, although they are 6 years old, and I would hesitate to recommend them for power-user functions (video production, music production, etc.)
It's a mistake to think that these machines can be the equal of modern MacBooks simply because RAM and HD can be expanded. GPUs have progressed way beyond the GPUs in these machines, and that's a big deal, especially considering these are not retina laptops and retina is a huge advantage. Additionally, these laptops only have a 1600MHZ bus, and that's not upgradeable. Modern MacBooks have a 2133MHZ bus. The bus is the highway of connectivity between all components and RAM on the logic board, so the speed of the bus, in addition to the GPU, is a big deal.
It's also turned out that lack of expandable RAM hasn't been the catastrophic situation we though it would be 5-7 years ago, and that's in large part because laptops without expandable RAM also have very fast SSDs that plug straight into the board. When the laptop runs of of RAM, it writes to disk, and fortunately the disk is VERY fast, so it's far less noticeable. And also, 2012 retinas and above do have removable/upgradeable SSD modules...I believe it's only in the most recent iteration of the Pro that the SSD is permanently connected to the board.
In conclusion, it's great that the mid-2012s have upgradeable RAM and HD, but that's only going to get you so far. If you need a more powerful machine, you're going to have to go get a more powerful laptop, and the upgrades you can add to a mid-2012 won't save you. I personally have a mid-2014 retina Pro that I bought for $200 broken and was able to fix, and it's WAY more powerful than the MD102LL/A that I had been using up until 2 years ago (which had been starting to drag at that point). If you do buy a retina Pro, make sure to get one with as much RAM as you need, because that is NOT upgradeable. Also, avoid 2012 and 2013 if possible, because those years had some issues. It's worth noting that retina Pros have glued-in batteries, which is a pain, but I've performed several battery swaps at this point, and it is do-able. So again, not quite the catastrophic situation we anticipated.
All that said, the mid-2012s are still reasonably powerful and good for most tasks, and they are in the $300-$600 price range at this point, so if you aren't a power user, they still represent a great alternative to spending $1500. If you buy one make sure to replace the spindle drive with an SSD, because that is by far the most performance-increasing upgrade you can do. And make sure the RAM in there is 1600MHZ, because slower RAM will downclock the bus.
Another bonus -- the mid-2012s are the oldest Pros still compatible with the latest OS (Mojave). Whether they will work with a future OS is impossible to tell, but the fact that you can buy an expandable MacBook that is still current for $300 is pretty impressive.
Hope that helps!