The Legislative branch could not intervene in such an event, except by impeachment of the court. Because the Legislative branch is split 50/50 in the Senate, they are constitutionally paralyzed from intervention, as we observed in the case of saving Trump's presidency from the election fraud. The Legislative Branch would not be able to intervene in a SCOTUS vs POTUS showdown.
If SCOTUS allows the illegitimate unelected POTUS to pack the court, that is only the first step of Biden's plan. Next, he will implement court rotation, and the current SCOTUS members will lose their seats. It would be the abolition of the Supreme Court itself. SCOTUS has full legal & moral authority to declare POTUS illegitimate.
REMIND SCOTUS THEY ARE COEQUAL WITH POTUS!
Best incentive: If the justice packing means that the existing supremes have to pool their current salaries to be divided by 12 or 15 or whatever number becomes the new level. After all, there is no budget for the newbies.