Major and Cameron weren't always expected to go the distances they did either.
Smith and Miliband never became PMs now did they.
TL,DR: Don't count out BoJo yet.
I think though that what might be the case is that 2019 was to Boris was what the 1992 election was to Major and what the 2015 election was to Cameron, and then what comes next is to Boris what Black Wednesday through the 1997 election was to Major and what UK voting to leave was to Cameron. That said, I'm not counting Boris out either - at least for now.
That was a big reason Tony Blair won big in 2001 is a lot of Tories just stayed home.
Was this the case? I was under the impression that Blair won a lot of crossover support and a lot of Labour voters stayed home. I think the turnout was down in safe Labour seats, I think Sunderland South was something like 48% compared to the nationwide 59%, but that's just one seat. I do know that the swing to the Conservatives was much higher in the safe Labour seats than in the "New Labour" seats.