I think it’s easy to misunderstand the difference between appropriated money (money someone is allowed to spend) and spent money (money that’s used, adds to the debt, etc). If you do something like allow a large portion of unspent allocation to roll over to future years (like… 95%) then some departments/agencies/etc will save up huge stockpiles of allocation - like places that will need to replace a satellite or renovate a large office building, or buy a new piece of land, or… Etc. This doesn’t add anything to the national debt, but makes for a scary headline - which is practically the worst thing for Congress.

The likely outcome would be lower spending, but there’s the faintest possibility that every civil servant in the whole government simultaneously decides that this is the year to renovate their office building, buy new computers, upgrade the coffee machine, and stock up on printer ink… And that would be very expensive, that year.