I think it's fair to say that the last established/big Premier League side to be relegated was Newcastle, and they managed to bounce back both times (although the second was a close run thing to win the Championship).
They had arguably a poorer squad and an even more terrible owner in Ashley who basically had the club up for sale constantly.
Difference with Spurs I feel is that they have huge resources and have been inexplicably included as part of the Premier League's "Big Six" despite very little to show for their outlay. This is why the neutral is either perplexed by or mocking their demise.
Would they come straight back up? Probably, but I reckon that like Newcastle they'd struggle for a few seasons then go back down again.
I'm not sure where the truth lies about Ashley being a terrible owner.
The fans loved him when he took over from John Hall who'd somehow become reviled despite sinking a large part of his personal fortune into the club.
Ashley ran the club like a business and refused to bury them in the tailspin of debt demanded by the 55,000 deluded screaming entitled jaw-dees who felt that success was their birth right despite not having won any silverware for decades.
He appointed the kind of popular high profile managers that the fans demanded (Keegan, Shearer and Benitez) who sometimes succeeded yet often failed (can't remember who was in charge when they finished 5th, I do remember that Benitez was somehow still a hero after taking them down).
His model of scouting players who appreciated in value during their stay is now the blueprint for sustainability for many other medium sized clubs. Alan Carr's Dad was mostly to be credited for that.
Throughout, he was always clear that the club was available for anyone else who could take it forward and eventually the Saudis did.
But in conclusion, yeah, Spurs are doomed.