I am not sure how this makes sense since the best way to support a fleet is just ... more ships.
Incorrect- the best way to support a fleet in combat is aura effects, which affect the effectiveness of the fleets. You
could increase the fire rate of a fleet by 10% by building another 10% of ships at thousands of alloys,
or you could build a 250 alloy Command Center. You
could increase your first strike capacity by building more first strike vessels,
or by building a 50 alloy communication jammer to slow enemy speed by 20% and emergency disengatement chance for more kills.
As you said yourself right now static defense are quite easy to overwhelm in the mid to late game, while fleets are mobile and have strong offensive AND defensive capabilities, especially with torpedoes and launchers, as was previously mentioned.
And this is why Starbase aura effects are their strategic niche. The reason that Titans are called mobile starbases isn't their combat ability, but their combat
support.
Resources wise it's almost always better to build ships for support than static defenses. If the intent is, as the ftl modules suggest, to hold a hyperplane junction, then star bases should be competent at doing just that. Fortress worlds seem a bit of an aberration to me. They are almost impossible to take AND bombard and would be impractical to maintain in any logical economy as their only purpose is to station billions upon billions of defensive troops.
...and that's that point.
Fortress worlds are indeed a non-productive asset, which is why you almost never build them in the early game and only build them when you have the economic surpluses to afford a large, non-productive military expense. But they are a strategic investment worth pursuing
precisely because they are almost impossible to take or bombard into submission, which requires the enemy significant time to to do. Time to move, or rebuild, your fleet after a loss that led to the point where you are being attacked, rather than the attacker.
And this last point is the key one, because to insist that starbases should be able to fill the same stall roll
without fleet support begs the question of what you expect them to do
when they have fleet support. Is seiging a starbase supposed to be a multi-year affair? Why?
To have a fortress world in the first place, you are making a strategic consideration that
you can not afford enough fleets to never come under attack in the first place. Whether it's because your empire is too wide, your forces too dispersed, too weak, or whatever. Therefore you are giving up economic gains (the opportunity cost of not using the planet as an economic world), and doing so knowing that any investment could be hard-countered by a Colossus or warp drives,
because time in the face of an inferior local fleet is worth more.
To argue that that stall role should be the role of the Starbase is to argue that a starbase should be able to hold out for years- or decades, depending on the previous investment- against a foe who could already defeat your fleet.
Which begs the question you still haven't addressed of... why?
IMHO they are the biggest problem with stalling the game right now. Maybe limit the fortress building to just one per planet would fix the issue.
Alternatively, you just don't understand how to effectively use the strategic tools available to you.
