The Nightlord is a victim of lore and fluff. She also is very much a jack of all trades and very very much a master of none because of it, she's a good assault ship, an adequate cruiser killer and a poor carrier, the only thing she does well is yeet Mech's at planets. Lore wise we've got the Lore pretty much saying that save a few Clans (literally 2) the Clans themselves don't actually value WarShips and see them as ill wanted solhama for the most part. And then you look at those lists of ships available and go "If they didn't want them why do some Clans have 20+ ships?"
And why would the Clans keep ships that are 600+ years old (Aegis class) around when parts for them are going to have to be near impossible to get without setting up a factory SPECIFICALLY for these ships? Basically its like the silly idea of going "Why not reactivate the Iowas?"
You would have to build EVERYTHING on her from scratch. We don't make armour like that so you'd have to make armour forging for it, the gun pits for 16-inch 50cal rifles are probably long gone, and there's no place in the world that builds those kinds of engines any more, so you'd have to rebuild those.
or spend billions basically rebuilding her and then its not worth doing.
And the Clans are not going to spend the resources needed to bring the Aegis up to modern standards and their hulls would be deteriorating and no matter how good their tech, they can't stop entropy and they can't stop metal fatigue that would probably have meant that the Aegis class would have been scrapped prior to the Amaris Civil War (at which point the oldest Aegis would have been 394 year old at the outbreak) and a replacement made. And yes I know that basically McKenna's made a lot of ships obsolete, but the SLDF also built the Luxor and Riga II to escort them and these three classes were probably a forerunner to a next generation of SLDF ships that would see a new Destroyer built etc.
So instead of keeping ancient warhorses around where you have to hand build every damn replacement part, why didn't the Clans build new ships as this would actually be cheaper than maintaining some truly ancient ships that would be as reliable electroincailly as 1970's italian sports cars that you see for sale in news papers.
https://www.youtube.com/watch?v=GuCff8nCxBUSo building new ships to replace the old, recycling them makes sense in terms of pure logistics, and thanks to advances in electronics and the like you'd probably reduce maintenance, crew manning levels etc. But this is Battletech and its not the 1st time the Lore don't make any bloody sense when it comes to the Clans beyond THEIR WHOLE SOCIETY WOULD COLLAPSE! IT MAKES NO BLOODY SENSE!
Of course if the Clans had half a brain they'd use the Creche system and Iron Wombs to pump out people at a huge rate, sending off jumpships to explore, colonize, exploit every damn planet and system in reach. Its got a violently flaring red dwarf with useless planets but rich asteroids, orbital habitats HO! colonise the habital worlds they found on the Exodus trail, fortify them, use asteroid mining etc and build up to defend yourselves and have the room to grow instead of squabbling over the hardscrabble worlds of the Pentagon and Kerensky cluster.
But I digress. We know the Clans can build new WarShips, the only thing stopping them was very inconsistent lore.