KE: As you know, even MMT does not claim that it is always prudent (“affordable” in ordinary language) for the federal government to increase its spending without limit.
For one thing, technical feasibility and resource availability impose potential constraints, as you yourself have pointed out in this thread.
Second, under certain circumstances, inflation can impose a potential constraint. No serious MMT writers deny this, although they correctly point out that conventional economists often exaggerate how close we are to the inflation constraint,
Third serious MMT writers acknowledge that the US government has self-imposed constraints on spending imposed by law. They may think those self-imposed constraints are ill considered, but as long as they are there, they must be taken into account in setting budget priorities.
In short, serious MMT does not deny that the government faces budget constraints. The question of “affordability” is always on the table as a legitimate topic of discussion, just as it is legitimate for MMTers and conventional economists to disagree about which constraints are binding in any specific situation.