I think once private equity firms buy up restaurants and (over) expand them to the degree the market is over-saturated, that's when they quality rapidly declines.
@sansucre is totally right. It's the selling-out/expansion/franchises that kills the quality of what were once upon a time great restaurants.
It's happened with The Ivy when ownership changed and suddenly now almost every town has an Ivy. I still go to The "real" Ivy and always have excellent service, with decent (but not outstanding) food but it's not the place it used to be. These days we always get invited upstairs to the private members' club. That would never have happened in the old days when there were far more important people dining to invite us peasants up there. 
The days of chatting to celebs in the Ladies are over. I'm sure Victoria Beckham will be relieved. No more 3 month waits (if you're lucky!) for a table on a Fri night. Sad really.
Same with Wagamama. I remember the early days of queuing for hours down Lexington St to get a table, when there were only 2 branches, and the food was fabulous and the communal seating quite ground-breaking. Now it's generic pan-Asian, bland and full of screaming kids.
Also happened with Gaucho. The original Swallow St branch (when it was the only one!) was wonderful. It really raised the bar for steak in London. My friends and I spent a lot of time and money in there. Then they expanded and it became a race to the bottom. Looks increasingly like Hawksmoor is going the same way. Desperately hoping this doesn't happen with Goodman and Blacklock but....
And Patisserie Valerie, what a downfall....so sad that now they sell their rubber cakes in Sainsbury's when decades ago they were so special and one of 'the' places in London to go for tea and cake.
I also remember having a birthday dinner in Fifteen. When there was only the Hoxton one and Jamie still did some cooking there. The food was excellent, which was just as well as it took us a good few months wait to get a table.
Of course is all so last century. 
I rarely have a bad meal out these days but that's because I refuse to go to chains and stick to local independents as much as possible. However, when lockdown lifted last summer, we did make the mistake of going to The Wolseley for a late lunch/early dinner after being at the National Gallery. Tables were still distanced so it was lovely and private, service was fab, food was pretty poor. And I had the worst scones I've ever had in my life. They must have been almost as old as me. Terrible!
Also had rubbish food at anywhere owned by Drake & Morgan (especially The Folly at Monument). Same with Carluccio's (they used to be so good when Antonio was still alive and on Neal St) and Pizza Express (although the Hong Kong Soho branch rocks!).
And finally a dreadful, slow meal at The Marksman in Hackney. Rave reviews from people, God knows why. Service was appalling but also invisible for much of the time. Staff lied to us about running out of dessert. It was a small menu but most things weren't available. Took 3.5 hours to eat a 3 course meal, including 1 hour to be served drinks/water!! Never again. And they're not even a bloody chain.