I think it's just that the bandwidth of the microwaves is not much by today's standards. It was capable of 150,000 simultaneous telephone calls, which is something like 10Gbs. It may be that modern numbers were higher, but possibly not much as you have to share the microwave spectrum with other uses. In a fibre you get the whole optical spectrum to yourself, so you can achieve much higher speeds. You can also run many fibres in the same ducting and increase bandwidth that way.

For backup / resiliency you just run more fibres via different routes.

We don't do backup or resiliency anymore in search of "value for money"

Fitzrovia is laced with fibre; the entire area is/was host to a large number of broadcast service providers, media companies, telcos, etc.

Running out of fibre is not likely in that part of London.

From memory at the time there was corrosion on the fixings and it would've been exceedingly expensive to repair/replace.

Maintaining redundant fibre links (which exist) would likely be cheaper than maintaining a decades-old microwave system (I would wonder if they even have the appropriate spectrum licenses anymore, anyway).

Cheaper to replace fiber during the day when most of the customers are out, than to hire a rare specialized technician to maintain it maybe