If returns cost them so much they'd do it now. But they don't: It is so much cheaper to produce the garment as it is that it won't change. They already make money from folk buying products and not bothering to return them.
No more free returns just means more consumers will be wary of ordering. The better fit stuff won't happen until actual laws are put into place and/or places start punishing manufacturers by refusing to sell their products on their platform, regardless of who is reselling them.
I very highly doubt it will affect whether or not returns are free, though.
Is it predatory on unsuspecting buyers? many times. Is it worse for a slice of the economy? Yes.
But free returns is unsustainable. Hence it eats into quality of products sold.
Even in the US, there are countless things you cannot return.