It could lead to those situations that you have described, but equally it could not. Our players are contracted on mostly long-term contracts so they will only be sold if we receive an offer that is acceptable to us. Berahino should show us that. So, losing our players will only be on our terms. In terms if we lose them, then losing Bale, Modric and Carrick has resulted in what downturn in performances i.e. league position? We have obviously shown that we can replace them and maintain a league position, despite numerous managerial changes in that time.
In terms of dropping to a position where we are unable to attract the quality of player we are now, I disagree. We will still be able to pay the 6th highest wages in this league and as is continually made out on here and I think perhaps by you (apologies if wrong), the smaller teams than us have managed to strengthen to catch up with us.
Mid-table does not in itself put a club on a downward spiral. Saudi Sportswashing Machine is one example, fine but is that a result of being mid-table or is it a result of the club hiring managers like Sam Allardyce and the general lack of direction and strategy. Look at what Swansea are doing. There is an example of a club on an upward trajectory. Liverpool fell into midtable not so long ago. They haven't fallen into a downward spiral as you suggest.