I feel like these discussions are ever more pointless, as you guys respond with little but surface level analysis and sound bites.
The real question is why Republicans are holding it up at all?? Why don't they cooperate with Democrats, the way Democrats cooperated with them during the Trump years?

I mean really? The Democrats "cooperated" in spending more money, which is their goal at every turn. A better way to phrase this was that Trump and the Republicans compromised by granting additional spending for Democrat priorities to get to bipartisan support.
You never see Democrats offering to support any Republican priorities to get their votes. Or can you list out the big compromise offers the Democrats are making?
You have a "reconciliation" bill on the table that includes fundemental and enormous changes to how the country works and the laws that govern everything, no part of which is to the benefit of the Republicans (or the country for that matter) and you never hear anything about the details because the media is in the tank and knows exactly how unpopoular much of what's in the bill actually is around the country (not even small majority against it, big majorities against some of these things), but somehow "democracy requires" that the minority position become the law of the land by any means necessary.
After all, this money is needed to pay for all the programs that the Republican Congress authorized during the Trump years. It has nothing to do with any new programs, especially those which have not yet been passed by Congress. So why threaten to wreck the economy over their programs? 
By government programs you mean the $3trillion Congress provided and Trump accepted to fight coronavirus, or the $1.9 trillion more that Biden authorized?
I mean 2/3s of government spending is "nondiscretionary," but ask yourself why any of it is. With only 1/3 (including things the federal government was actually formed to do like provide for the military) on the discretionary side, there's a never ending conflict on how to spend it. The very fact that the Democrats are constantly pushing for new "entitlements," which then become "nondiscretionary" puts a lie to the collaborative process. You want a true collaboration then cap what can be spent on Democratic priorities and Republican priorities at 50%. Make them responsible for budgeting their own priorities instead of giving them an unlimited backing and incentives to underspend on what's important.
It's like local school boards. Every time they want more money they threaten to fire the art department, the music department, to shut down sports, when none of that eats up any significant part of the budget. Every time they get more money they don't make those priorities safe, instead they lock in a new contract with the teachers union to eat up the budget "excess" and put themselves back in the same spot 2 years later.
Quit falling for stupid lies.
The main reason Democrats don't want to use reconciliation is that they are still working on their main reconciliation bill. Should they just throw that away, or rush it through, just because Republicans are too ornery to pass a bill to keep from wrecking the economy? Sounds like blackmail to me.
And using a "reconciliation" bill to pass transformational legislation in a 50/50 senate and closely divided House isn't blackmail? The people gave a mandate for compromise and the middle road and the Democrats are abusing a process to act like they have a 90/10 senate and 100+ majority mandate in the House.
And it's not like the old days where its one shot only. The parliamentarian already ruled the Democrats can use reconciliation 2 or 3 times. McConnell's "demand" is that they burn one of the illegitimate "extra" reconciliations that they plan to use to abuse the system even more.