I did not mean 1 on 1 but rather as part of a fleet.
Well, yeah, the capital ships weren't alone. The point is that they were a massive part of your early fleet composition, so how two capital ships fared when directly facing off was very important to the outcome of early fights.
Lets take e.g. this scenario for instance (even before rebellion): You jump into enemy roid with 30 Cobalts and a Marza. Enemy has 30 Cobalts and a Sova. The 4 bombers would not be able to take down the Marza by themselves, but while you bomb you force the enemy to either engage the Marza or lose the planet. If they attack the Marza one could go for the kill on the Sova while the Marza survives (HP and Armour wise).
This was not the case in the late Trinity meta. The Marza had been dominant in the earliest versions of the game, but then this patch happened. All the carrier capital ships got heavily buffed, and the Marza was losing badly against the new carrier meta. This was not theory-craft, in fact at the time my theory-craft was that carrier rush would be a fun gimmick but wouldn't upset the best options. That was proven very wrong when carriers crushed the old meta and replaced it. The Marza was still a strong capital ship on paper, and did see occasional use as second capital ship, but the meta was hostile to it as first capital ship and it was rarely seen in that capacity.
The fragility of carrier capital ships was offset by the fact that they weren't beholden to weapon range. They could simply micro outside of your attack range and just kite you if you tried to chase them. Their fragility really wasn't a problem in practice in a meta where the only good way to fight their bombers was by going with a carrier yourself.