Here's the thing:
1: Typical Dn at that level runs MCr 120 thou to 140 thou. Given a DN with good agility (a must in my book, given the lethality of meson spinals), a fighter needs both a bridge and at least a Computer-8 to have a 1/36 chance of hitting the Dn; a bridge and Computer-9 about triples the odds. The resulting fighter costs in the neighborhood of MCr160 to 180 and comes in around 80-ish tons. Credit for credit, you end up fighting the 200 Kt Dn with 700 to 800 fighters, aggregate 60 Kt. It's the dang computer - they cost a ridiculous amount, and the fighter player ends up spending a whole lot on a lot of computers. If the DN is heavily armored and has a decent array of sandcasters, the 700-to 800 fighters might get 5 or 10 shots per turn past the sandcasters, of which 1/36 does any damage. They're losing 2-3 fighters per turn to put one hit on the DN about every 3 or 4 turns.
2: I don't fight single battle. Wars aren't fought in a single battle. I fight campaigns. And in a campaign, the Dns can eat fighters, then break off and go home when the enemy Dns take the line. The Dn spends 1 to 4 weeks in repair, the fighter player spends 32 weeks building replacements - and he spends much more than I do. After one or two victories, the enemy finds himself with too few fighters to penetrate the sandscreens unless he concentrates to overwhelm particular star systems - and that gets into grand strategy, where both sides are doing their best to battle with advantage instead of at even odds.
3: I don't screen with fighters. I screen with escorts. For the price of 7 or 8 of those fighters, I can build an agile size A escort that the fighters can barely hit - and the escorts are about ten times as likely to hit fighters with their F9 missile bay. At TL15, they can be pretty heavily armored, so they tend to stand up well when the enemy Dn's take the line. So, where you'd field 700-800 fighters per Dn, I can field about 100 little 1800-dTon escorts per Dn, and they don't take crits every time a F-9 missile hits them. Vulnerable to the spinal, but a very difficult target. And, between their F-9 missile bay and a couple of pulse lasers of their own, they're almost as good at harvesting weapon hits as the fighters - and they're more likely to be able to limp home for repair after the exercise, and I don't end up with a stalemate where two lines of fighters can't hit each other. Moreover, because the escorts stand well in the line, I can run with battleriders if I choose and be able to screen their recovery by the tenders.
So, my TL15 Dns aren't troubled by fighters. Lower tech DNs, that gets a might harder.
o:
