While multiple years of OS support would be nice in this form factor, not being able to use 5G data would be a big negative in terms of this being "futureproofed".
5G is extremely overrated. LTE is easily capable of 200Mbps. I’ve personally seen it... but very few people have. Have you? (relevant link that demonstrates over 500Mbps on LTE: https://www.iphoneincanada.ca/news/iphone-11-pro-max-lte-spe...)
Until our towers have enough network backhaul to consistently max out LTE to a phone when that phone needs it, 5G has exactly one benefit: less signal congestion at major sporting / concert events with high densities of people. The carriers would say “marketing” is another benefit, probably.
Just having 5G won’t make the internet any faster in most situations, since the towers are backhaul limited... they aren’t limited by LTE.
Even 200Mbps is more than enough for 99% of people's phones for the next 5 years, especially since that's like double the average home internet speed in the US. The 537Mbps seen in the article I linked to is just excessive by today's standards... and that just requires current LTE.
“5G” also refers to two completely different technologies, which most people probably don’t realize. One of them doesn’t even seem very different from LTE, and the other requires towers for almost every single city block.