I don't think Slayer playing 8 or so reunion shows (with another one booked for late this year) negates the fact that they're retired. Because Megadeth will probably play occasional shows to mark certain anniversaries etc. too. The retired period being when touring is no longer work and they just play a few shows more purely for fun.
Well maybe not seriously, but for a bit of fun once he gets over some of his ability declining. He could even have another guitarist show up and just do vocals at some point.
Sure he could have some one else play the guitar, he could have someone else sing, he could have someone else make silly comments. Nothing is impossible