All recent blizzard games are built to be easy on the surface to attract a wider range of players hopefully converting a few into more hardcore players. This was a philosophy stated at one of the blizzcons; I don't remember which from.
Honestly I'm enjoying the change it feels like blizzard is aging with me as I fall into more real life events such as multiple jobs, I can still log into bnet and have fun for a couple hours a few days a week. Games you can play every so often are becoming a really big thing and blizzard seems to be trying to tap into that market for heavy casually timed gamers while still offering something for people who have more time to dump into a game.
I'll be playing overwatch and probably spending money on it even if it's f2p. I like supporting blizzard and I'm really looking forward to this game, as I feel it will be another game I can play without much free time.
There are so many popular games with "cartoony graphics" look at: dungeon defenders, tf2, torchlight, heroes of the storm, league of legends, smite, battlefield heroes(not really popular but a good game nonetheless), warcraft, starcraft, hearthstone, and the biggest of them all world of warcraft. The graphics on this game isn't what is going to be it's downfall. It'll be lack of a progression system and gameplay, I can guarantee there will be hardcore players in this game like any battlefield or cod game. As for overwatch becoming an esport, we'll have to see I honestly didn't think HOTS was going to even compete with LoL, but the community really trashed that game.