I've thought about the long-term future - and viability - of WoW since MoP. It has generally been my opinion - until recently - that the franchise would eventually have to do WoW 2.0, beginning fresh with maybe some way to transfer old WoW accounts to maintain some continuity. The issue as I saw it was the need to attract new players; a lot of us who have been playing for several expansions are getting older and playing less and we know the playerbase is much smaller than it was at its prime. While WoW can't really hope for those numbers of subscribers again (barring substantial expansion into untapped markets), the game needs to attract new players and WoW has become to unfriendly to newcomers - something a 'reboot/sequel' would seemingly achieve.
However, the increasingly effective means of upgrading the games' systems and graphics has made it easier to patch in, and use expansions for more major revamps, the changes needed to keep WoW viable. This makes part of the need for a 'real sequel' moot - we can get totally new systems, entirely new graphics engines, etc. without every destroying the existing WoW realms.
The other major issue has been leveling. Now with 120 levels, beginning a new toon - especially as a new player with little gold and few if any heirlooms, is extremely intimidating (and one free max level isn't that helpful for a new player who will more than likely choose a class that won't end up being best suited for hem). Fortunatetly, it appears Blizzard is very aware of this (and how it also affects alts - with each new expansion, beginning a new alt becomes all the more tedious). Even before BfA we've seen an expansion sized change with the change to leveling. I tend to think this is just the beginning; I think we'll see continuous work on consolidation of leveling, the story, old zones, etc).
I also think BfA is doing right by moving more away from toons being demi-gods to them simply being great warriors again. Moreover, the new subraces appear to be just the beginning of a huge expansion on choices for customization of existing races, the addition of many new sub-races and the easy addition of new races (Vulpera certainly look like they are made to join the alliance with the Sekarath finally giving the Horde a snake-race soon). They also seem more interested in increasing spec/toon customization.
It appears a mild-reboot can occur via an expansion or two and a slew of patches.