The constructors of the Lewisham extension, CGL Rail Plc (City Greenwich Lewisham) came to the end of their concession, apparently 24 years long, in 2021, and it was handed back to TfL. Meanwhile TfL had been checking it all out for some four years before this, since 2017, so should have had a complete picture of all assets - TfL of course have the largest Lifts & Escalators department in the country, so are not short of understanding. It's now 2025, a further four years on from the TfL takeover. Contrary to some misleading statements about the change, they were not "bought out", they came to an end point in the concession agreement. It was particularly an issue that the agreement payments were based on passenger usage, and in 2021 this had dropped right off due to Covid, so the concession owners had made a large loss in that last year and were glad to be shot of it. Maybe TfL were hoping to extend the concession, but they weren't entitled to expect it.
I don't know what maintenance contract CGL had on the escalators, but it assuredly would have ended in 2021 when their concession did, and TfL should surely have ensured the continuity. Things like manufacturers' assurances on availability of spare parts would have ended in 2021 at the end of the contract. Possibly CGL had some contract condition that if they got an extension of the concession, the maintenance contract would also be extended (such arrangements are normal), but of course CGL were not extending their contract.
Comments on lower architectural standards are bounced around, but the whole thing of doing a PFI centres around a cheap (it can be) way of doing infrastructure investment when the principal doesn't have the money. Typically you get a straightforward utilitarian project, it works, and you are saved the architectural ego-trips and such like that seem to inflate public projects (Jubilee Line Extension take note). Notably nothing on the Lewisham extension got the architects awards at industry annual events or anything like that. It was similar to the original DLR, done for £77m, an absolute bargain, could never have been afforded otherwise, was criticised for being excessive compared to the initial bus solution, but once it proved successful was then criticised for being underspecced. I have used Cutty Sark since it opened (in its first week, just days before the Millennium) and can never remember the escalators being failed before 2021.