There are so many ways they could have innovated to stay relevant but has anything changed in years there?
They could have hired professional reviewers. Yelp reviews are notoriously bad.
They could have let you choose what you prefer, example (quality vs quantity) and given you custom ratings or only included reviews from people that share similar tastes.
Following up on the last idea They could have tried to auto cluster reviewers. (people who rated these 5 places as having good taste also liked ...)
the could have scanned menus so you could ask "where can I get a pepperoni pizza at 3am" instead of at best being able to look up "pizza" and figure the rest out yourself.
How about "where can I get a table for 10". How about partner with as many restaurants as possible so I can ask "who's still got a table for 4 for steak and win between 7 and 9pm tonight"
And maybe nothing would save them but I at least for me reviews make sense on Google maps. That yelp couldn't see their audience was going to dry up seems no different than newspaper classifieds and Craigslist. Their quality certainly isn't better so ATM there is zero reason to go there.