I'd agree that the quality of men has declined, but I saw a podcast that tried to explain it by saying women these days don't offer men anything either. These are the comments in the podcast:
The modern woman is strong, independent, and self-sufficient, so support-wise doesn't need a man.
Many can't/won't cook, cleaning is off the table, and being a housewife is seen as 'oppression', so that element is out too.
'Body positivity' has also become more of a thing recently too, meaning that his glamorous supermodel believes she no longer has to look good (or his idealised perception of it); ~50% of uk women are now obese. So she's not going to stay attractive for him either.
And historically, sex was somewhat seen as an obligation, and that thankfully is out the window as well.
However, women still statistically aim for men who earn more than them, who are financially secure.
So, what does a man get out of the relationship? He's not getting the 'traditional' woman who needs him, nor a housewife who takes care of him, nor a woman who looks after herself, and he's probably not getting any sex either... then, when the relationship breaks down, he'll lose the majority of his assets too. If men don't 'appear' to benefit from a marriage, why would they put any effort into one?
It goes both ways, of course, and it's a feedback loop... the less attractive men are to women, the less women put in the effort, which feeds the 'sloth' of men. Men aren't helping out, are less ambitious, sleep around... I don't think it's one gender's fault at all.