There is nothing wrong with turtle tactics, in fact most founding queens are doing exactly that, claustral colony founding (digging a hole, close it from the inside, lay some eggs and wait). There is also quite some ant species that have developed turtle tactics so far that there are specialized ants with huge heads to close of areas (Camponotus truncatus).
For the same reason it is nonsense that the queen is sitting close to the nest entrance(s) all the time and there is currently no way to move her "deeper" into the nest, as the majority of real colonies would behave naturally. This is also player unnecessary "player steering".
I'm not too fond of the idea to actively prevent players from using a tactic they like. Ants do fill excessive space, so why should we not be able to do so in the game? For me, it is also a matter of aesthetics - I like certain looks and it's my colony, so why should it not be as symmetrical or chaotic as I wish it to be?
An implementation of filling tiles with earth should be trivial, really
And yes, in that case it probably should also be possible that enemy creatures can dig into a nest, after all you can "fix" the intrusion way and should have some kind of "early warning" before a breach. However, real ants/termites would use already existing connections over digging new ones.