I have to agree. He was an excellent tubthumper, he could really draw the crowds and he did get the traditionally disengaged younger demographics interested in politics. Indeed Labour under his watch achieved a 40% vote share in 2017. But as was demonstrated all too brutally, that isn't enough to win a general election in the FPTP system.
The problem was that these votes were concentrated in the big cities but many of those didn't help the party in terms of seats overall, so a lot of those extra votes were wasted under FPTP. In the 'red wall' seats, it was a very different picture as most young people with any ambition had left these mostly small towns for better prospects in cities and university towns, thus leaving behind an even higher proportion of disgruntled ex-Labour voters who were more amenable to the messaging of UKIP/Farage/Johnson. Dominic Cummings was all too aware of this and exploited it ruthlessly.
Under FPTP, it's not just how many votes you get but about where those votes are cast.
Besides that, Corbyn unfortunately didn't have the leadership skills required to get the job done. The PLP and the party machine were working against him from the very outset (as outlined in the Forde report) and he was too weak in dealing with his enemies, which was his undoing.
And little did people (the vast majority of people anyway) know that a global pandemic was around the corner. Had it been a Labour government in power in 2020, it would have been severely challenged. I don't think there would have been the overt corruption of 'VIP lanes' etc but I think there may have been even more resistance from the tinfoil tendency than there was under the Tories.