Personally I'd want there to be two very high quality, proven strikers brought in before we consider getting a few million for Defoe. If Defoe's happy to have to fight for his place and maybe "only" be an impact sub I'm not sure what the point of selling him would be. What would we get? Something like 7m? So would we rather have a) 7m; or b) a player who's proven, who loves to play for the club, is settled and who epitomises the old cliche of "natural goal-scorer"? If Spurs need a goal then I can't imagine many other players I'd want on the bench.
Sign some player who's happy to sit on the bench, not gonna want higher wages than Defoe, is not going to cost 15-20-odd million and will actually offer more coming off the bench when we need a goal? That's a tough ask.
There have been times when Defoe was by far my third-favourite striker at Spurs and was just waiting for someone or other to offer 10m. But it'd take two very good strikers to be brought in before I'd want to see the back of him now.