Are you just speculating or has Thomas spoken about retiring within the next 2 years? He's 70, which isn't exactly young, but unless he has some health problems that I'm not aware of, my guess would be that he could probably count on another decade on the bench.
Thomas has stated on several occasions that he wants to actually enjoy his golden years and doesn't plan to stay on the court so long they have to take him straight to the care home upon retirement. In fact as far back as 2005 there were rumors that he was seriously considering stepping down. However, when the democrats took over the Senate in 2006 and then Obama won in 2008, he decided to stick it out until a Republican was back in office. Which is why it's a bit surprising that he hasn't retired already. Most court observers believe he is trying to wait a year after Kennedy to minimize the disruption to the court. Of course that's a risky strategy because if democrats take over the senate in November, he could be stuck on the court for another decade.
If Thomas has ANY THOUGHTS on retiring I hope he does it NOW, he will be a hard one to replace HOWEVER so was Scalia AND Gorsuch was a one in a million find for Scalia!!!!
I have no doubt that Thomas and Kennedy discussed the matter at length and agreed that Kennedy would retire this year and Thomas next year. Both would only want to be replaced by a Republican President and Republican controlled Senate. If Kennedy had not retired this year, then Thomas would have.
The risk of the Democrats retaking the Senate is always there, but is much lower this year than most years simply because most of the Senate seats up for reelection this year are Democrat seats in states that went heavily for Trump. Even if the Democrats manage to take over the House and win lots of state elections (which the Democrats and media expect and I think is doubtful), mathematically it is unlikely that they will pick up a net gain in Senate seats and will most likely end up with a net loss (my guess is a net loss of 3 Senate seats).
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.