Please explain why you think he'll be significantly better off long term?
He's down £8-10m minimum as things stand by not signing a new contract, which by the end of his contract will be closer to £12-15m. De Bruyne and Sterling are also currently on significantly more each week and there's absolutely no reason why Sane couldn't have been on similar money at this stage if things had gone to plan.
Even if you take all the financial issues away and backtrack to the "ego, lack of desire, lack of character" narrative you're now suddenly trying to push as the main reason; why would you think him leaving is so disrespectful?
If I didn't have the confidence of my employer I would want to leave too, I wouldn't want to work for an employer who didn't appreciate me and if anything it would severely knock my confidence, never mind send my ego into overdrive. You would leave just like every other poster on this thread in those circumstances, especially if you felt like you had given everything you could to prove your worth which after two record breaking seasons in a row Sane must surely have felt like he had done.