so then you rotate your fleets in and out. engage with one, hold others nearby to pin any other approaching fleets(AI has the same limit), and before your fleet takes significant amoutns of damage you havbe them jump out whilst a fresh fleet takes over. functionally the same or superiour to doomstacks - just a ton more micro.
and it's not like doomstacks are a problem to begin with. fleets are fairly slow - if you'd doomstack one gigantic fleet and attempt to smash your opponent, they would be able to evade that fleet due to their information advantage, be able to send smaller fleets to smash any space-based thigns you have in your empire, prevent invasion fleets from reachign their worlds. if you break off your doomstack to challenge them you defeat a small part of their navy at best that's quickly replaced while the other empire still has most of it's space intact. even if you get a decisive engagement, starports are able to pump out ships in significant speeds enough to mount a defensive, so your damaged fleet that most likely lost a sizeable portion of ships can't exactly brute-force their way through.
basicly, there's no logical or gameplay reason to dissallow doomstacks. based on what we've seen, they're not a problem to ebgin with, nor does it make sense to limit them in space. winnign wars are not about destroyign fleets, it's about destroying the ability to wage war in the first place. battles are but a means to an end, and doomstacks suck at harrassing larger parts of space, or dealing with multiple smaller fleets.