I haven't checked, but is the SSD in the current MBPs replaceable, in case of failure, but only with Apple stuff? Or is it fixed?
It is replaceable, but good luck getting the parts.
RAM on the other hand changes. My 2006 MB ran awesome with 1 GB of RAM, in 2006. In 2010 I needed 2, and that was maxed out. My mini came with 4 GB, I think? It was alright for a while, but after some OS X updates it was sluggish. It now has 16 GB and runs wonderfully.
That is not that much of a problem nowadays. E.g, the 15" model comes with 16GB RAM, which is the max supported by the CPU anyway. The 13" and 12" come with 8GB, which is sufficient for casual use* for the next few years and have the option of 16GB, which is again the max**. The things will change once we move to DDR4 or some other tech that can deliver higher RAM densities. Then, 32GB becomes a possibility.
*There are two main reasons why you need more RAM: 1. increased requirements of common algorithms and 2. higher-quality content. For 1., the current programming techniques involve a lot of indirection and therefore need tons of RAM. But we have hit high peak some time ago and the trend is now actually towards economy (e.g. Swift, Rust). As to 2., there was a massive boost in content size requirements with the advance of retina displays (quadrupled data size), but again, it has more or less platoed right now as the assets are already very high quality (we might see anther boost though as wider gamut becomes standard). In different terms, we have now reached a level of quality (both in software development and assets) where throwing more RAM at general tasks doesn't really help or is not nessesary. Of course, there is still an increase, but it slowed down. Also, earlier we had a very small standard RAM installations , so every incremental bit was a huge deal. But even now, few years after 16GB became a common thing to see, an average user won't see any performance increase with going from 8Gb to 16GB. I am not talking about specialised users who need a lot of RAM, but those probably won't be happy with a laptop to begin with. After all, to efficiently utilise a lot of RAM, you need the bandwidth as well as processing speed.
**There are high-dencity DDR3 modules, but they are very expensive and only supported by the newest Intel CPUs.