Agree with OtherJen and WhiteTrashCan. Biden pulled off something really impressive in this election. Aside from Barak Obama, Biden won a larger share of the popular vote than any democratic presidential nominee since Lyndon Johnson all the way back in 1964. He flipped states like GA and AZ that democrats haven't won since the 1990s and, while he only won GA by thousands of votes he won more enough other states by at least an order of magnitude more than thousands of votes that he could have lost GA and still won with room to spare.
Also agree with the above posters. This was a VERY impressive win. (I also want to point out that 1992 defeat of George HW Bush almost certainly would not have happened either, had it been a 2 person race without Ross Perot splitting off a ton of the conservative and independent vote.) The AZ and GA flips are quite astonishing. I expect those states to backslide for an election or two, but if they move into purple territory after that, the entire electoral map is going to be upended in the future.
Re: whether the Dem coalition is strong, that's an entirely different issue. The majority of Dem voters (let alone independents) are not nearly as far left as the activist, highly visible wing of the party, which is why Biden ran away with the nomination despite all the sound and fury of media focused around more 'exciting' further left candidates. Biden was helped by left-leaning voters' raging desire to unseat Trump in particular, and the suspicion that we needed a more 'boring', centrist candidate to do that. And those suspicions turned out to be correct.
This election has pointed out some very flawed assumptions that Dem election strategists have been working with, mainly that minority votes can't be wooed away by a wildly racist 'populist' GOP candidate. I found myself quite surprised by this, but in retrospect it should have been really obvious, given that Trump won white women easily in 2016, despite being a disgusting sexist, serial philanderer, and possibly a sexual predator, running against an incredibly qualified white woman.
The Dem coalition is always more fragmented than the GOP coalition b/c it includes more disparate groups of people. In addition, the Dems lean heavily on identity politics as part of messaging, which I think backfires on them just as often as it activates them, which can add to the fragmented nature of the coalition. The Dems also want to actively govern and create policies, which is much more difficult to sell to the average voter than is the GOP's message (protect guns, outlaw abortion, dismantle laws, cut taxes).
The Dems always have inherently a harder row to hoe in order to get elected, and the Senate is by design incredibly weighted against Dems taking control, even in relatively favorable years.
So...yes, Dem coalition is weaker. Odds are that Dems rarely control the Senate in any given year no matter what the overall political climate is. That makes Biden's win even more impressive IMO, regardless of exactly why and how it happened.