Specifically the claim that 'beam combat will never happen if they can disengage' is not true. There are two distinct interpretations of that which seem apparant to me at this time.
First one is that they will never engage in the first place because they see a disadvantageous engagement and choose to not take said engagement. In order to do that the player would need to reliably predict that that is the case, and I think it could be reasonably claimed that neither the AI nor the player will be perfect at this. It would also require that the defender have no targets that they are obliged to defend, which is also unlikely (you would probably have to defend your main population, if not any other critical installations you might possess). In general though I think it would be a relatively good thing, because it means a star could change hands without a fight even necessarily taking place. You show up with a fleet, present an unwinnable situation, and the enemy chooses to withdraw (and is actually able to reliably do so). That type of scenario has I think been lacking from aurora for quite a while but is entirely realistic and in my opinion can be fun because locating a large percentage of the low tech enemies fleet isn't necessarily an instant game over for them, meaning the conflict can be drawn out and there is a chance more things will happen.
The second possible interpretation is that the ships trade blows, and then the side that is currently losing decides to run away. I think thats fine, because you will probably kill some of them anyways since most likely there are several mobility kills if nothing else, and even if not then you have inflicted damage that they now need to go repair, thereby inflicting a load on their repair and logistical infrastructure, as well as temporarily taking ships out of action, which will make a difference to their overall ability to continue fighting.
e: In general I would argue that if a damaged fleet is able to withdraw, then it allows a much more logistical form of warfare. If both sides can usually withdraw before a ship is outright destroyed (barring an unlucky mobility kill) then that means you would potentially be trying to win by straining their logistics to the point that they can no longer make enough of their fleet ready to continue offering effective resistance. This is a much more fun and complicated concept in my opinion, because you now have new design considerations, regarding how you build your ships, and how you prepare for a conflict. Deep wells of replacement components for repairs might become more useful, for instance.