Wouldn't that leave them in the same position as the Republic in the 3130s, i.e. surrounded by powerful, numerically superior enemies?
History has not been kind to states that try to hold Terra and environs. It's the most vulnerable position in the Inner Sphere. The key to holding it seems to be diplomatic: you need either assiduous neutrality or incredibly good diplomacy, and on top of that a big stick to disincentivise people attacking you. The Hegemony had its technological edge and Aggressive Peacemaking strategies, and even then it was conscious enough of its vulnerability that it invented the Star League to secure its borders, and invented and deployed the most powerful space defenses in human history, in a time of peace. ComStar was rigorously neutral and threatened to shut down the HPGs. And the Republic was only possible in the first place because everyone was allied with it, and even so it desperately wanted everyone else to disarm.
Terra and environs are by their very nature vulnerable. You have half a dozen or more neighbours, at least several of which are likely to hate you at any given time, and once you start to fall apart, everyone has an incentive to invade and snap up what they can. Terra is not stable.
A Clan could abandon their entire OZ and try to make a 'Clan Hegemony', so to speak, based around Terra. But the Clans are noticeably bad at diplomacy, peacemaking, or neutrality; and the tech gap is almost gone. A Clan Hegemony might stick for a while in the Dark Age, since all the great houses are tired and devastated, but you can't hold Terra through force alone, and neither can you through the fear of force.
If not for the fact that their entire culture demands it, I would be inclined to suggest that trying to conquer and hold Terra is a strategically terrible move for any Clan to make.
