That assumes they can get 50 Democrats to vote for the specific language in the bill; which is far from guaranteed. Not all Democrats want to do the exact same thing about healthcare.
For a particular bill, however, they might peel off occasional Republicans, if they’re actually given a chance to vote. The 2K stimulus as a standalone would probably have passed if McConnell had been willing to bring it to the floor; more than one Republican said they were in favor of it.
Because she’s not overall a Democrat, even if she agrees with them occasionally.
People do occasionally switch parties, even in midterm. But they don’t do it to vote on one specific bill; doing that would leave them in the position of shortly wanting to switch back again, and they’d start to look pretty silly (and would be unlikely to get any of the perks of good committee positions, etc., as neither party would trust them to stay put if they switched repeatedly.)
If you want to get rid of the filibuster, you don’t make someone stand up and talk to stop a vote on a bill they don’t like. That’s just going back to the old filibuster, not getting rid of it. Instead, you just change the cloture rule to be a simple majority or even just 50 rather than 60.
One of Schumer’s foremost imperatives as Majority Leader will be to bring bills to the floor that will wrong-foot Republican Senators up for reelection in 2022. That is, legislation that pits one segment of their supporters against another. So for instance, bringing forward a trillion-dollar infrastructure bill that fiscal conservatives will scream about, larded up with projects that are critical to communities in FL, WI, NC, PA, etc. Each vote that he can force Republican Senators into that will piss off some segment of their base becomes a campaign ad against them.