I'd argue his rooting wall is actually very good for the Warden fantasy.
It's slow movement and very large radius makes it a very potent disengaging tool, similar to Braum R or Nami R. People can stand behind a tank to avoid the roots, but the ult's slow speed plus the relatively long size of the roots means they'd need to wait behind before actually coming out, lest they be caught by the back end of one of the roots and be rooted anyway.
As a Warden, this makes the rooting wall a very powerful peeling and disengaging tool. When used creatively, it also makes for a powerful ganking or engaging tool; imagine ganking bot and sending it perpendicular down the lane towards the wall, and the enemy having no way to cross that distance without being rooted thus forcing them to either eat the ult or force them into a fight while they wait for it to pass. Or, imagine sending it headlong into a Baron or Dragon attempt from the river, thus CCing at least 3 enemies and heavily disrupting their attempt and setting your team up for an advantaged fight.
Those of course require unconventional, creative uses of the ultimate so it still retains and is best used for it's core purpose of disengaging and disrupting fights in progress, rather than initiating them itself.