By this logic you should max out everything - why not get an 8Tb drive? You might need it at one point, and you'll be able to sell it for more.
The thing is - people can reasonably predict their needs, unless you're dramatically changing your line of work. People act as if suddenly there will be this big shift - omg, files are getting bigger, you will run out of memory, etc. This may happen - but that goes for anything - you really might realize that you need a faster CPU or a bigger drive, so why not buy that 8Tb drive? You never know! Of course, most people are not buying these 8Tb drives (even 4Tb ones) because they can reasonably predict their needs. Same with people buying M1 Pro CPUs instead of M1 Max CPUs because they don't need the GPU horsepower. But how do they know? GPUs are used for a lot of things - what if tomorrow they discover their work requires more GPU cores? Better make sure and buy it now! (In fact, as silly as it may sound, I actually find it more plausible that in the next 5 years some people might need extra GPU cores more than additional RAM, but that's a totally different topic).
It's just the RAM thing that is a complete mystery, where people keep thinking they need more for whatever reason. My guess - RAM amount used to be much more important for everyday things then it is today and it used to be this big limiting factor (if you run out of RAM, you don't get to use your app). So now we have a situation where even journalists like Joanna Stern tweeting something like "Write You can never have too much RAM on my tombstone" - which is kind of a bad advice to give. And when computers slow down, people immediately think RAM is the reason, not some background process or software issue.
The reality is - if you're ok with 16Gb today.... you'll be fine in next 5-6 years. There will be exceptions, but there are exceptions for everything. And in 5-6 years, when your tasks become more demanding and you could use more RAM, it won't be like your computer will stop working, just that you'll realize you need more. But you will also need more CPU power at that point, and maybe there will be a better screen that you can use, etc. So might as well get a new computer then.
P.S. Speaking of reselling, it really depends on the asking price - because usually you will ask for more if you payed more originally, and people buying used computers don't usually spend that much money. Cheaper used things sell more then expensive used things.