The evolution of player base for ``random lobby'' games is not something new and has been experienced by various titles. Since ESO is only making baby steps in this category, things can go harsher than for some more developed and mode-oriented projects (some of which suffered very harsh fates). The fact that classes and play styles are far from being equal in ESO adds up to the following problem.
At the current point, those players who get punished (no matter how good they are) are
--- solo players
--- players with builds which are bad for scoring in BGs (a DoT build who gets his kills stolen, low mobility builds in CTF/DOM)
(a player can be in more than one category)
Even though wins will happen from time to time, these people will face defeats more than other folks, and, safe a few tryhards, will get at best bored with the overall inability to change the outcome. The outlined categories of people will be visiting BGs less and/or leaving eventually.
In my opinion, this is already happening, as I see much greater numbers of magsorcs in BGs than in Cyro, and they are often in premades.
The result of this outflow may decrease the BGs population significantly, and it is already small enough to fight same people in multiple rounds. It may well lead to the fact that the winning premades will get bored with the recurrence and also leave BGs in favour of Cyro.
One solution would be of course to open BGs to everyone. It would drastically enlarge the playerbase. But, I think, the problem with battlegrounds lies at the foundational level. 4v4v4 with premades possible will bring further imbalance. One option would be passing to 6v6 or even 8v8 with no premade group greater than 2 (or 3) participating, with compulsory counterbalancing group on the other team. Perhaps we will need greater (not necessarily less detailed) maps. I do not claim that I hold a complete answer, but I believe that the closed beta testing was barely sufficient for the BGs. They may well need a thorough overhaul.
I would really hope that the developers can learn from other titles and not allow all this to happen. Fingers crossed.