> pretty hard-working but over time become less so
If people start to cruise, they may get pressured by hard-working peers and management. Of course, those who are very good may get away with it as FAANG reward impact and not work, but there are many strong engineers there, so it's very hard to stand out. And those who do are incentized to climb the ladder rather than cruise.
> it doesn't make much of a difference how hard they work
It's the opposite. Those who manage to have good evaluations are rewarded.
> seeing highly-motivated people having their potential sapped by working on contrived pillars instead of using their talents to make cool and useful stuff
There are tons of cool and useful projects in these big companies. Basically any field of CS you may be interested in is there.
My experience is that people work hard when they get the job. After one year, they realize it's hard to sustain that pace and that they barely meet the expectation with all the hard work. Some leave, some get managed out, and those who stay many years are productive individuals.