Totally disagree. Being the oldest #1 is literally nothing compared to another slam. He'll become #1 anyway if Nadal fails to defend most of his 4,000 clay points. Who could possibly put a ranking ahead of a slam!?
Recency bias, imho. You obviously bought Sampras' argument that it's all about the slams. Well it is--now. But in 50, or even 20 years' time? (If Fed's GS record is out of reach, the ATP will need another narrative to sell their next, fledgling GOAT to the masses.) Any way you look at it, getting to #1 is a much, much bigger achievement than winning a slam (and ever more so at 36). No way he's going to let that pass, imho. I can definitely see the lure of #20, but #21? That's a case of #20 + #1 >>>>>>> #21, and basic maths be damned, if you ask me.
For the record, unless it's totally out of his reach, I also think he'll go for #1 at 37 (ie some time during the summer/fall) *and* YE #1, too. He won't make it a priority, but if the chance is there, he'll try and take it.
Edit: plus, you seem to imply that playing Dubai would mean missing Wimbledon, whereas is about as far removed from Wimby as can be (yes, Rotterdam would be even better, and I've said so repeatedly), so playing both shouldn't be an issue--like, at all.