Froome‘s legacy, to me, seems already clear: he will be remembered as the four-time TdF winner, who won all GTs, and who crashed heavily afterwards, to return much weaker than he was in his best years.
But he returned, and stayed a professional cyclist. It‘s clear to everyone that this guy loves bike racing. He loves bike racing, and accepts that he has become a domestique. Although he won the Tour four times, bike racing, to him, is more than the TdF.
Completely different to, for example, another champion: Jan Ullrich. Ullrich would never have continued his career, under circumstances that Froome has to face now.
Results-wise, Froome nowadays gets humiliated by his rivals, when he climbs the mountains of the Tour in the grupetto, at best. I, however, think that his popularity has grown, now that he shows so much character and passion to just stay WT cyclist. Especially the French seem to admire him for that, and I know why: Froome, after all, is a champion, on brighter and also on less bright days…