The landslide successes of the right in recent years have made me very sceptical of such optimism. When centrists and leftists were confident of having a winning formula, they often failed (although in some instances it was down to the electoral system as well.).
Looking beyond the UK (I don't live there), I see the following basic problem in many places: The right is far more united, as their mass support is more sociologically coherent & more receptive to nutty symbol politics and identy rather than appeals to rational political interest. The cultural divide
@Adisa talked about is the perfect environment for them to thrive, and they of course foster it further.
The (more-or-less) liberal center and social-democratic left are much more divided, as they're more focused on their quite large practical differences, which I guess also reflect their sociological bases to a degree. (Not to say they aren't ideological, but their identitarian offers don't appeal to groups large enough to win.)
So a typical constellation of our times would be two blocks of halfways similar* strength - one can act as a block when push comes to shove, the other as a shaky coalition at best (and it doesn't work). This problem seems especially bad in places with an effective two-party system.
Going back to the UK, where I think all of this largely applies: I'm not sure how the "rational politics" Starmer offers (in the eyes of his proponents) should make a fundamental difference there. Although I'd appreciate it if they did.
* (Not everywhere, of course - just a generalized image to make a point)