It's been over 10 years (14 to be precise) and Battlespire hasn't been released for free? That's sad. I really want to play it.
Arena wasn't released for free only because it was 10 years old. It just happened to the first game at the 10th TES series anniversary. Daggerfall was 13 years old at the 15th TES series anniversary when it was released for free.
Maybe it's for the 20th TES anniversary?
Indeed, that's quite likely.
If they release a game for free in 2014 at the 20th TES anniversary, it will be Battlespire for sure. And then possibly Redguard in 2019 at the 25th TES anniversary..
Morrowind won't be free anytime soon as it's selling rather well on Steam and still run as intended on modern OSes so no DOSBox tinkering is required. It would be really stupid of Bethesda to give away something for free when they still make money out of it.
HA! Only problem with that theory is that it isn't part of the series, it's an additional game (notice no Elder Scrolls: Roman Numerals).
I don't see why you consider that as a problem?