When you consider the combined might of all of the marketing arms of these ridiculously large companies or people involved, it'd be a travesty if the publicity was an issue. I think that will convert into viewership if the teams are good enough and they should be.
It seems likely that Seoul will use L-H, with their sister team replacing the current roster in APEX, Shanghai will field a relatively competitive Chinese team, Immortals have a solid base to work from with a genuinely strong team, and NRG and Misfits both have time to replace people as necessary to form a top team worthy of being in there. The two newcomers in Kraft and Wilpon will have the resources to either buy Rogue/EnVyUs (or possibly another Korean team but seems unlikely), or form their own idea of a superteam from Korean/Western pieces. That's before any other owners sign up as well with their opportunities to bring in good teams.
Anybody who fields a shit team to OWL will get roasted. Even though the level of competition is unlikely to match that of APEX in the past three seasons, where the best region played the best few foreign teams, it's gonna be global, competitive, and well publicised. It's also not gonna have any other more competitive league in the world other than APEX, which has stopped taking foreign teams so loses that charm.
Why would viewers not watch? It'll be promoted through in-game items, client announcements, social media everywhere, hopefully in the game itself (though not announced at all yet). I feel more confident now than I did before this announcement. Not saying they'll get hundreds of thousands for every match but APEX finals got something like 150k last season and this will be pushed harder with teams more people in the West care about.
As for your questions to locality robokun, your issues actually make no sense! The games are all happening in LA for the first season, so there's no problem with lack of attendance at local venues. When those are implemented, it will be Overwatch fans/players turning up to watch, same as any other esport - they are just ALSO likely to capture some small chunk of the local wider esports or gaming fanbase.
Would people in LA turn out to see Immortals despite their three Korean players? Fuck yeah of course they would! People aren't put off by watching others of different nationalities, and if they were then American CS:GO major tournaments (with majority European/Brazilian teams and players making finals) would flop in attendance. They're fans of the game, the competition, even the teams and players without that local connection. The locality of it only adds something to those who would otherwise be uninterested. To a 'hardcore' fan it doesn't add much but it crucially takes nothing away.