My 2c's worth.
I'm an inexperienced player wandering in the seas here because of the borderline unplayably woeful server speed I get in Warplanes and I've found submarines to be enjoyable to play as a new casual player in coop battles - circle high-traffic areas mainly at periscope depth trying to be a nuisance with no danger of anyone telling you how to play - the extra physical dimension (up & down) is pleasing compared to the surface ships.
I think that on this basis they're a sound business move for Wargaming... stick some on the tech trees, have some to buy I think you will expand a player base because they are so casual user friendly: sure I don't make much impact in games but I don't get killed very often compared to other lines & have fun & that's the point, right?
I see the worry of the long-term player, but I don't think change will affect a mass player base here as much as on WOWP as long as this game retains its friendly coop mode - despite its own problems - because it is just so newbie friendly. Part of the problem for the potential player base with Warplanes (compounded by structural frustrations and a dreadful server) is that it's a SIGNIFICANTLY harder game to play for the inexperienced casual because you will get outclassed more often, k/o'd more often, and lose more often with nothing to show for it which means it gets a bad name.
Have fun all!