In a few ways, quality has improved—that first set of minis was pretty awful in terms of paint quality. Things have definitely gotten better than that since then.
Second, WizKids is a separate company. Even though the two product lines are clearly related, they are separate products by separate companies.
Finally, “continuously improving quality” (as a mission) has a lot of facets. “Quality” can refer to manufacturing errors, which might be invisible to the customer. Also, there will always be an upper limit to quality, and diminishing returns on improvements; WotC and WizKids could certainly make their prepainted minis “better”, but doing so would shift the price-point away from their target market.
Long story short, I don’t think you should be expecting WizKids to improve the quality of paintjobs very much, unless you’re willing to pay a lot more.