I think Beta cards will dip during Chaos Legion release, but will spike back up to an ATH after Chaos Legion is finished printing.
Some cards will definitely become less relevant. Like, imagine if a 'new' Cerberus (In the sense of a self healing Fire tank) was printed? Its value would plummet (though it would still probably be expensive) There's some definite risk there, but I think a lot of Chaos Legion is going to focus on interesting counters and meta changes to Untamed card sets and metas specifically.