There are a number of posts in different threads about the question of not flying the last segment (or more) or a RTW ticket.

I see this as two seperate but fairly simple issues.

One is what the rules say. RTW tickets have complicated sets of rules to try to ensure that they are used by global tourists not cheap business people like me. When you get your ticket issued you will have abided by the rules. If you get a ticket reissued, it will still be within the rules. But if you fail to return to your origin (or certain nearby points layed out in the rules), then you no longer have a valid RTW ticket. The correct solution in these cases is for the airline to reprice what you have already flown as point-to-point fares, then subtract what you paid - less 10% penalty for many originations. Anything left over, in the unlikely event there is anything, would be refunded. Or far more likely, the airline will want to collect the fare difference, which could be many, many thousands on a first class RTW with 18 or so sectors already flown.

OK, the second issue is - will they catch you and will you get penalized. The answer to that is unclear, but it seems pretty unlikely you will get caught if you are just dropping the last sector or two, and you aren't making a habit of it. But for sure if the airlines catch on they will enforce the rules.

It all comes down to how comfortable you are with the small but potentially expensive risk. Like driving way too fast on deserted interstates in the USA...