I understand his thinking but selfishly, I don't want him to leave. The chances of finding someone more conservative than Thomas AND then getting that person through the confirmation process is basically nil. Even if the Republicans pick up Senate seats this year (and that is likely), the confirmation battle would be brutal for a strong conservative. There are enough squishy Republicans who would likely defect that this battle would be quite uncertain.
The problem is that he is not immortal. He will have to retire someday (or will die in office). Waiting beyond next summer is a hell of a crap shoot.
I think that Trump will will reelection in 2020, but at this point we do not know for sure that he will even want to run again or who the Democrat candidate will be. Plus after this next election, I don't see any the Republicans increasing their Senate majority anytime soon.
So Thomas can either choose to retire next summer, when he knows that Trump will replace him with another conservative, or risk having to wait another 4, 8, 12 years or more before there is another conservative President and Republican controlled Senate.