Real long term thinking would be a modular signalling system where you could reliably replace one section in (say) 48h, so you could close for a weekend and reliably re-open with whatever tech we will be using in 2070 in place.
I might be a bit jaded on this. I spent about 8 years nursing these systems to keep them working because there was no way we could close a TfL line for 3 months and completely change the signalling system and make it faster, safer, more reliable, self driving etc. Even having moved to finance, I am routinely delayed on the way to work when the (new) CrossRail signalling system conflicts with the (old) system on the same line for different trains.
C'est la vie I guess, I understand discounting so basically anything that takes 40 years to be useful is NOT worth doing...
Stockpiling £50,000 of computers might well be cheaper and lower-risk than upgrading a signalling system.