Honestly? There's no way to answer that question definitively.
From a marketing and merchandising perspective, so long as he stays relevant, it's possible that Ohtani pays for himself without even considering his on-field contributions. He may well create a generation of Dodgers fans, and set up a pipeline for future Japanese talent. Not to mention all the jerseys he'll sell.
While on-field value is easier to calculate, it's still tough. I haven't looked it up in a while, but not too long ago people were using roughly $8-9M as the value of a win on the free agent market. Taking the conservative number there, he'd need to be worth ~ 8.75 wins per season to be "worth" $70M per season. But that's not what he's actually being paid. So we need a better idea of what the net present value of his contract is to figure that out. If he's really only making $50M per season, he'd only need to be worth ~6.25 wins at $8M per win. Or as few as 5 wins if we end up at $10M per win on the market.
But even that isn't accurate -- as teams have largely tried to stop paying the below-average players the full value of their contributions. Instead, they're trying to concentrate wins on the roster -- a guy like Trout was considered to be worth more money just because it's hard to get that many wins in a single roster spot. Well, Ohtani is poster child for consolidating a bunch of production into a single roster spot. He basically creates a whole bench spot by being both the DH and a member of the starting rotation. So even if the market says $8M a win or whatever, being able to consolidate that much production into a single roster spot may be worth $12M or $15M.
So long story short -- we will never be able to accurately say whether he's been "worth" the contract or not unless we see something approaching either 99th percentile or 1st percentile outcomes here.