The issue of merc's retiring is actually a pretty real one. That said, knights in history were also pretty wealthy, and had the same issue--why fight when you can sell your weapons/armor and retire, or go rogue and just take over a town with your small force? In history the answer was that someone with those weapons/armor you sold them to cash out could just take your land/money since they now have weapons/armor. So keeping some equipment to defend your self makes sense, but then you need a household to maintain it, and land to support that household, and you are right back to needing to go on campaign to get money to protect money.
In my headspace, personal owned mechs make a lot of sense as knights of old, who needed land to rule over to support their military force, and whose military force was instrumental in keeping their land/acquiring money to live on. So even a merc or pirate company should have territory somewhere they are tied to, with each mech having a familiy back home they are supporting/being supported by. In that sense, a 10 million c-bill payout is nice, but if your household is 100 million net worth with a 5 million cbill mech being part of that estate, then we see why mercs dont just pirate a 300 million dropship at the first opportunity to go rogue--the lance has 400 million in estates in their families name, and a 300 million split is a net LOSS when their families assets get seized so a few people can go live out their days on a pleasure planet. Pirates found kingdoms for this reason in btech Id bet--to have a territory to maintain and support their war machines, which then needs the pirates to raid to support their kingdom.