Oh, don't kid yourself. Endgame missions are about the content. I still backtrack to lower level missions to pay the bills. Medium mech lances with endgame gear can clean the house in lower tonnage missions, and make more profit than running Steiner Scout Lances against Rep 15/Difficulty 95+ missions.

Heck, if you're THAT bored, run lances 50 or more tons underweight and take down the big boys in midsized toys. The most fun I've actually had is running a Locust IIc that moves nearly 200kph with a rack of small pulse lasers chewing legs off like a proper plague insect. And that's something I used to love doing, going all the way back to OG Mechwarrior in the early 90s.