Why do you feel this way?
Especially when we are led to believe that there is a fifth era.
I foresee the series continuing for a long time as they (Bethesda) have no shortage of material to work with/create.
I agree here, I'm sure Bethesda can still come up with new ideas for Elder Scrolls sequels, and I don't want the series to end until Bethesda starts running out of good ideas for it.
Though if the Fifth Era is really going to be like what the lore we have on it suggests, I hope the series stays in the Fourth Era, I read the Loveletter recently and it doesn't really sound like what I look for in the Elder Scrolls series at all, Bethesda already has the Fallout license, so there's no need to turn the Elder Scrolls into post apocalyptic science-fiction too.
As to an Elder Scrolls game spanning all of Tamriel, I'm not sure if it would really be feasible to do that with the current technology available, while maintaining the level of quality and detail I want from the series, and I wouldn't want an Elder Scrolls game spanning all of Tamriel if it would come at the sacrifice of detail and quality. Better to get one province at a time but done well to have a game that has all the provinces but makes all of them underwhelming.
And I don't want an Elder Scrolls MMO and doubt I'd play one, if it were made.