The animation in TAS isn't bad because it's old, it's bad because it bad. It's below the standards of even 1950's Bugs Bunny cartoons.
Er...do you realize that the Warner animation studio had a large number of artists working on the short subjects produced during its heyday in the 1940s? Do you realize animation houses of the TV era deliberately streamlined the process to be cost effective and efficient, but that does not automatically mean inferior quality (otherwise early
H-B, like Jonny Quest
or The Flintstones
would not be praised for technical as well as story merits).
it's difficult to think of a modern example of TAS's particular animation style.. Even everyday average animation like Family Guy, and American Dad are far superior in technique and quality to TAS.
Most modern animation is quite poor, with several companies employing a form of CG and in other cases, flash animation, which (in my view) is not much better than moving construction paper figures connected by paper clips. Over the years, many of the series aired during Cartoon Network's Adult Swim
programming block used this kind of animation, which is on par with the worst of H-B's 1970s work (when the studio farmed work out to Australia).