Hi OP. Over the last 25 years, we have lived in Bethnal Green, Pimlico, Putney, Barnes, Chelsea.
I wouldn't say everywhere in London has gone downhill. There had been huge investment in some areas - eg.
The whole area around Battersea Power Station - unrecognisable compared to 20 years ago.
The whole area around London Bridge Station - when did that happen?
Spitalfields / Hoxton area - transformed
Also, I went back to where we used to live in Bethnal Green / London Fields and couldn't believe my eyes. Totally buzzing. The flat we sold for £80k when there was no more than a pub and a laundrette on the street, is now selling for £900k - it's super cool cafes and matcha lattes all the way there now!
Chelsea and Belgravia and that whole zone retain a sense of 'upmarket.' Still lots of character, generally uplifting vibe. Lots if backstreets and I still always stumble across something I never knew was there.
High St Ken is not great, but the residential streets leading off it are probably posher than ever. South Ken is the same as it always was.
Hammersmith (especially near the tube) has had a lot of investment inc. initiatives to reduce pollution. Cycle lanes, pavements widened, shiny new office blocks and apartments and a bit more 'cafe culture.'
Parsons Green is still pleasant.
Notting Hill / Portobello is a bit rough round the edges, but my teens love it.
The West End - Oxford St I couldn't say as not really been shopping there in years, but I wouldn't describe Covent Garden as a depressed area? Soho much better than 20 years ago. Same with Chinatown, Carnaby St - the whole zone really. We went to see the lights in Bond Street recently and the whole West End / Mayfair area was buzzing with people. Great atmosphere and felt very safe.
On the other hand -
Putney. The High St has def gone downhill. Above street level, some of the buildings look near collapse. A lot of fast food type places have replaced the shops that used to be there. Looks noticeably 'grimy' these days - which is odd because the river area is still lovely as it always was and the residential streets all around are more expensive than ever.
Wimbledon Village is still posh in the horsey kind of sense, but actual Wimbledon town centre is very bland.
Richmond is still ok (The Green and Riverside are beautiful), but lots of shops seem to have gone out of business - inc. Wholefoods (the horror!) and the department store that used to be there.
So that's my summary! In general, I think the way we use high streets has changed - ie. people shop on high streets less, but go for other reasons (socialising, eating out, etc). So 'destination' high streets, or those serving areas where people have higher disposable income, have survived because the shops turn into Insta-worthy restaurants, boutique-end shops, etc. But where people can't afford that type of lifestyle, the shops just stay boarded up.
Two other points (while I'm at it) -
Yes homelessness (and homeless gangs) are more noticeable in the last 10 years. But this is also true in many other European capital cities. Particularly Paris.
When you travel outside of London, it can be a shock to see how depressing other parts of the country can be. I think most people in London have no idea really what it's like in your average U.K town.