Insofar as Trident renewal is concerned, I find myself on the horns of a conundrum.
On the one hand, any sign of weakness on NATO's eastern-flank, might encourage some of Vladimir Putin's more adventurous inclinations, which would be a very bad move, imo.
On the other, it seems abundantly-clear to me, that Islamic fundamentalism is a much greater and more immediate threat.
On balance, I would probably renew, but push suppliers to reduce their costs by 1% - even if it was just for the first qualifying revenue-year, it would be a bundle - and put what is saved to good use, bolstering our underfunded Intelligence and Security services.
As far as currency of the technology is concerned, this applies across a whole host of industries, and is a particular risk when it comes to major infrastructure investment. You have to start somewhere though, and the skill is in ensuring that there is a degree of flexibility available, when it comes to supporting future-state technologies. Notwithstanding the cost element, the world is far better at predicting these types of thing, than it was when Trident was first designed and built, and I think it's reasonable to think that obselecence isn't necessarily wholly built-in.
Regardless, that obsolesence risk is not a valid reason for not renewing, imho.