I'm not sure how to articulate it but for someone like Guptill, and to a lesser extent de Grandhomme, if you're 35+ and not getting game time, you're probably not far off international retirement anyway whether you want it or not. I'm all for Guptills decision which to me feels different to Boult's somehow. Guptill might be legit on his way out whereas Boult would continue to be selected for a few years yet.
I guess when they renewed Guppys contract they didn't necessarily know what their plans were with the white ball teams - if they did they shouldn't've offered him a contract in the first place.
Flexibility is good but imo being available for the national team is paramount. I guess it comes down to whether the player is otherwise a lock based on ability and performance, or someone who is looking to be replaced.
I would think in Guptill's case, he should really only be recalled as a cover for injury etc. like all other un-contracted players who end up in the Black Caps. Give his remaining contract to someone on the fringe, an up and comer, who deserves game time.
To me, he took the contract in good faith, believing he would be a fixture in our sides. That changed, as is the nature of sport. Nothing is promised to you. Now that he doesn't see himself in our squads, something he really hasn't had to fight for a lot in his career (apart from Tests), he's now pulled the pin for the T20 leagues. I'll be honest, I think it stinks - and I'm a huge Gup fan. He didn't perform in the Windies, he didn't perform in Australia, his form has been on the wane, so they dropped him. Justifiably. And now he bales.
I think a lot of our contract stuff has stunk this year. CdG quits on one, only because he got a better deal lined up in the Big Bash after the fact (he'd be happy to see a very decent contract amount roll in every week if he hadn't). Boult quits on one very shortly after accepting one, but gets superstar treatment in selection despite NZC often saying they'll give preference to contracted players...then they try and act tough on it in a non-consequential home T20 series. This one with Gup, and I suppose with Neesham he at least turned it down initially - although again, where's the tough stance on non-contracted players when the last six innings this T20 specialist bat has scored are 0,0,16*,0,6,5?
To your last point, I wouldn't pick Gup again, even if injury hits others. He's made it crystal clear where he's at. And I don't agree (you didn't say it, but I've heard it online) that he is owed some sort of loyalty after all this time. I'd argue the other way. NZC have given Gup so much, and I'm not sure pulling the pin because you've been dropped is a great way to show you appreciate that.