You sell cos this is likely the best time to do so. The alternative is to watch him decline while not having the funds to get an adequate replacement. Like it or not, we simply dont have the funds to run a transfer policy of holding on to ageing players.
The question of if the money would be spent on a good replacement is irrelevant, as we would have to get a replacement in a few seasons irrespective. In fact, having more funds makes it more likely to get it right than bargain shopping for unproven talent.
As an example, a sale of Son for £70-80m gives us the opportunity to get a replacement for £40-50m while still having £20-40m for another quality addition to the squad. That is far better than looking for an undiscovered gem for less than £20m