That's if Bethesda Game Studios ever makes another Elder Scrolls game again. It's looking more and more like Skyrim could be BGS' grand farewell to the TES franchise and that any traditional TES game made in the future may be made by a different studio altogether. Even then, it seems unlikely a new major Elder Scrolls game will even be announced before 2025, or at the very least before 2023 (BGS' next after their 2019 game). ESO was likely not made simply as a spin-off; ESO is probably intended to replace the singleplayer series for the next decade or so in order to keep the IP relevant and have something for the fans to play over the extended period of no new games. All in all, I don't know if I would worry too much about what BGS might do for a future TES game because I currently have no reason to think BGS is necessarily going to make anymore TES games, let alone get involved with any new TES projects (I imagine that currently might be ZOS' job).
That still doesn't mean that a voiced protagonist in a traditional TES game won't happen, though. In all honesty, if the developers of the next TES do decide a voiced protagonist is a must, they could probably get away with using about 2-6 voice actors and give the player a "Cyrodiilic accent", since Heroes with capital Hs appear to be metaphysically connected to something in Cyrodiil anyway. I don't know if a voiced protagonist would really work for a traditional TES game (poll here says NO), but I would take ten playable races that share voices to a degree over fewer playable races for a voiced protagonist. In my opinion, the ten playable races are too popular for even one of them to be axed. Failing voices across all ten races, I think they should consider doing a silent protagonist instead to preserve the playable status of said races.
The only thing we can figure is that if the next singleplayer RPG TES game is in fact going to be made by BGS, it probably will feature a voiced protagonist. It doesn't sound like BGS is moving away from voiced protagonists anytime soon. As stated above, I think the chances of BGS returning to the Elder Scrolls franchise might be slim, though, so I don't know what's going to happen with the TES franchise. Also, ESO probably has little to do with any lack of traditional Elder Scrolls games besides filling a void. It would more likely have to do with Todd Howard getting tired of doing Elder Scrolls games, and singleplayer TES games would in this case likely cease to be developed regardless of whether or not ESO exists.