There's no doubt that the club would be more attractive to a purchaser if it owned the stadium, but I don't think that the lack of ownership is the reason that there's no buyer at the moment.
My view is that prospective purchasers have found SISU impossible to do an economically rational deal with. After all, SISU would be selling a loss-making company with no assets to speak of, so talk of SISU "getting their investment (whatever that may or may not be....) back" simply don't make any sense.
This is, I think, what TF was alluding to when he talked about "not selling at the bottom of the business cycle". The problem for SISU is that this nonsensical plan to move to Northampton will simply increase their losses, as revenues fall much much faster than any saving they may make on costs.
This is what makes people suspect that the plan is all a bluff designed to scare ACL/Council/Higgs into surrender, but as each day passes it appears that whether by accident or design, SISU are actually going to go through with it - after all, TF "doesn't bluff".