Every single player of ours could be sold if that train of thought is taken to its logical conclusion, you know. The ones coming off a bad season could all be sold using the logic you used ('what if they never improve?'), and the ones coming off a good season could be easily sold by just reversing that logic ('what if this is their high point, and they decline next year?').
Don't get me wrong, I think Mason could do a good job in Eriksen's position, and we've got Alli, Onomah and even Dembele to cover in that role should Eriksen leave. But crucially, I don't think selling one of the most naturally talented players in our squad because he had a.....sub-par half season, is a good idea at all. Partly because of the seeming absurdity of the proposal itself, and partly because even if he is sold, I don't see the money being used to strengthen the team adequately anyway, not in this age of self-defeating, self-imposed austerity. In this era, if our best players must be sold, I'd rather they ask to be sold themselves, as opposed to just being sold in pursuit of mythical improvements to the team that will likely never occur.