You are correct, if next franchisee doesn’t acquire them, then previous owner would still have them. But highly likely new franchise wouldn’t have enough trains on day 1 if rejected them.
But presumably this wouldn't be an issue because the rolling stock requirement would be stated as part of the franchise.
If the new franchisee was required under the terms of the franchise to maintain the same level of rolling stock as Abellio, then this would be a franchise commitment and the stock required would come from the ROSCOs in the normal manner, thereby alleviating them from the need to buy out Abellio's additional stock. Alternatively they could simply revert to whatever level of rolling stock was previously specified. Either way, Abellio gets landed with the capital asset together with it's associated ongoing maintenance costs.
To come back to the OP's original question, where is the incentive to take this course of action? Even assuming that the operational penalties do not preclude it (platforming, maintenance and stabling, track access charges, etc), how would it benefit a TOC to buy it's own additional trains? All they are getting is additional costs that are not necessarily going to be defrayed against additional revenue.