Helps significantly. Assuming Democrats write off Alabama as a loss and build solid leads in Colorado and Arizona as they are doing now, they would only need two more seats to get to 51. Between North Carolina, Maine, Montana (assuming Bullock still runs), both Georgia seats and Iowa, not to mention reach targets like Alaska, Kansas, South Carolina, and Texas, it would be a very doable task and I think they would be favored, though not guaranteed, to flip the Senate.
In real life, it's still very possible, especially with Trump crashing and burning, but it's a tougher task than it would have been if Nelson had held on. Though it's not just Nelson - there have also been other close races in recent cycles that could have gone the Democrats' way, but didn't. Think Pennsylvania in 2016, or Colorado and North Carolina in 2014. Mark Udall's seat would have been Likely to Safe D, and Kay Hagan, assuming she doesn't die, would have also likely been favored.
They've also had their own share of narrow victories like Warner in 2014, which means 1 less seat to worry about this cycle, as well as Shahen's seat, which I actually thought Brown might win. In 2016, they got very lucky in NH with Hassan barely beating Ayotte as well as Cortez-Masto winning narrowly. In 2018, they had AZ, WV, MT all narrowly fall their way. It's not like one side can just have all the narrow races always fall there way. I think RN, AL is a guarenteed flip, even in a Bidenslide, and CO and AZ are on the weaker end of likely. You can't really make a logical argument for McSally winning, which is, sad, but who knows it could be WI-2016 all over again. Unless something goes terribly wrong for Ds, they should be able to pick up at least one of ME, NC, MT, KS, IA, GA, and AK, and don't really have any seats of their own to worry about other than maybe MI if Trump makes a comeback and James actually starts running a better campaign and lives up to the hype. -AL +CO, AZ, NC, ME is probably their easiest path, but having backups in diverse states across the country means that even if they collapse in a certain region, they still have a good chance at winning the senate.
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