Hi, what use do you foresee ? Modularity is nice, but the question is whether it is intended just because it is technically possible to put together … or because it is operationally necessary. Why did you reject Turris for your purposes ?
As a layman I recommend you to read the forum, you will find that I keep up to date the very first versions of Turris 1, which are on a different HW basis.
My Turris Omnia router is from 2016 (7 years old) and the update is running.
I myself have a 2014 (9 years old) Synology DS211j NAS that has already stopped update support. Yet Synology support is generally at a very high level.
Generally you can expect update support for HW max 10 years. The worst thing that can happen to you in 10 years is a failure of one module with no replacement and moral obsolescence. SW support (security updates) will probably be available.
If you insist on a guaranteed solution both in terms of HW service and SW support you have to consider a professional paid solution, where service and parts are extremely expensive (intel processor for office PC after 10 years is worth four times the market)
Personally, I would not plan the lifetime of a modular model for more than 10 years and especially I would think about the possibility of failure of one module and thus possible devaluation of the whole investment.