It was a great story of a star player from Toronto coming home to play for the Leafs in his prime. Plenty of players have had that opportunity, but very very few take it (at least if we're talking about legit top line talent).
However, we always knew the final ~1-3 years of the contract wouldn't look great, but the hope was we would go on some legit runs in the first ~4-6 years of the contract.
That hasn't gone to plan though, and there are very valid criticisms that signing him (when we had Matthews/Kadri down the middle) was the wrong move at the wrong time, and that we should have focused on better depth and defencemen.
Additionally, I'm pretty confident Tavares signed at 11M really set our internal cap structure, and contributed significantly to the difficult negotiations we've had with our other core four forwards.
So overall, none of the above is Tavares' fault, but I think the signing hasnt worked out quite as planned (though I do think he individually has lived up to realistic expectations you could have had of him up to this point).