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On a TL8 world however, if they've got an agent who can get access to some TLB or C systems and have contractors maintain them, they're not getting something that may be the latest development, but a tried and tested system that's been in use for hundreds of years if not longer. All the R&D costs would have been recouped centuries before, patents would have run out, and markets are in the hundreds not the tens. All of that could push the price of defensive systems down.
Yes, a pirate could sit outside the range and threaten to drop rocks on the planet, but if they could afford an anti-ACS scale system, they may be able to afford an anti-planet-killing missile defence system.
That's pretty much what I'm getting at. A (say) TL12 system could also be built for ease of maintenance (i.e. an export version). The days are long gone when avionics systems needed cutting edge hardware - you could build systems with off the shelf (albeit space certified) hardware, probably using off-the shelf components.
There is always a market for older military hardware, and supplies for it. For example, there is an outfit in Pakistan that still makes ammunition for 25 pounders, and there are a number of parties that make parts and upgrades for old jets like MIG-21's. A similar ecosystem of secondary market players could easily exist in a Traveller setting.
Given that Mercenaries are a thing in Traveller, getting crews to run these systems could be accomplished off the open market as well.
For my traveller universe (a bit lower tech than the third imperium) I did a bunch of 'old' Terran kit sitting around the TL11 range. This included various vehicles, missile systems, artillery and suchlike and were intended precisely for this sort of situation. The analogy I would make is old cold-war era equipment, Soviet kit in particular. This was designed to be idiot proof and easy to run in poor conditions. Not the best performance but still perfectly adequate if your enemies aren't equipped with expensive cutting-edge tech.
There were also some variants with third party modifications or upgrades with higher tech kit. For example, I did a light-ish grav tank with a B gun. This was designed to be landed in a cutter module so it had a configuration with relatively light armour that could be twinked out with add-on armour panels. It also had variations with a pulse laser or a TL12 fusion gun that was designed specifically as a drop-in replacement for a B-gun. Variants could also have upgraded electronics.
These designs were intended for use with mercenary campaigns and suchlike. As 'older' kit they were supposed to be representative of the sort of kit that you could find on the grey market. Stuff like this is much more useful for actual role-playing than the latest TL15 battle cruiser. It's the sort of thing you could reasonably expect to encounter as a band of shady adventurers looking to make a few bob on the fringes of the imperium.
