It would accomplish that, but it would also put users on vastly different scales of rep. Right now there's a reasonable rep distribution wherein there's a gradual and pretty common-sense progression from the most casual users up through Kyatric. (Ashley's obviously a very special case.)
Users like Yann and R0J0 provide continual, high-quality content on the forums: even if they only received 1 rep per "like" or "thanks," they'd receive a base of 2 points per post, plus on average 5-10 additional rep (because sometimes only one person would hit "thanks" and other times 350 users would all hit it).
If retroactively applied, I could see Yann going from 7K to an absolute minimum of 12K -- and maybe that's fair, but unless he drops off the planet, we would see a few users with stratospheric and untouchable rep. That's not necessarily wrong, but it's the sort of thing Tom and Ashley consider whenever they rebalance rep.
With the "Favorite User" option, there would be that huge initial jump as 80 people all add the ten most helpful people on the site, but it might fairly reflect the additional value they bring, and there would be a gentle but steady progression after that as new users join and recognize their contributions.