I wouldn't bet on it. Coinbase valuation will drop drastically when Tether bubble pops and bear market sets in.
If I were a betting man, I'd wager we'll see Tether play out similarly: Big spike in price (which we've already seen), massive drop to something reasonable based on actual demand and participation by real people with real money (currently ongoing, though surprisingly drawn out...the Tether trick seems to be working better than I would have believed, if it were pitched as a film plot), and then long slow climb back to a new all-time high (two years, perhaps).
Then again, at some point, something that works better is going to replace Bitcoin. Maybe the terrifying Tether crash will be the straw that breaks BTCs back. But, Coinbase/GDAX will be well-placed to profit from whatever that next thing is, as long as they're vigilant for new opportunities and careful as hell about security. Coinbase can only lose by making mistakes at this point. They profit whether BTC is going up or down, as they take (very high) fees regardless.
They have no banks, as far as anyone can tell. Every bank that they publicly had a relationship with has cut ties months ago. They haven't accepted new accounts from individuals since then, but the USDT market cap has exploded since then; they've been printing $100 million every day or so for the past few days even as BTC has been dropping.
If the money is coming from anywhere, it would have to be institutional investors...so, what institutional investor sends $100 million a day to a sketchy as heck looking little company with no audits, no accountants, no banks, and to buy an instrument that, according to the terms of service, cannot be redeemed for dollars. That really doesn't add up.
For me to believe $100 million a day is pouring into Tethers while BTC is overall either stagnating or on a downward trend (much more than when it was doing well and on its way to an ATH of 19+k), I'd need to see some evidence. They've promised audits, right on the front page of the website, for years...never delivered one.
I'd believe they're laundering money on this scale (which is mostly bad for them, when they get caught, though the market will be hit hard when their wash trades stop). And, I'd believe they're printing money on a fractional reserve model. This one would be even worse for the BTC market, because Tether has been buying roughly half of miner output for months now...that would mean half of the upward pressure has potentially been fictitious. How can that not be disastrous?
But, I simply can't believe they have 1:1 backing for over $2 billion. It just looks completely outlandish, given all the shady stuff surrounding the company and its founders. Extraordinary claims require extraordinary evidence, and so far, they've failed to provide any evidence, much less extraordinary evidence.
Someone sitting on a $2+ billion dollar fund that's growing $100 million every day can afford to hire a reputable accounting firm to look at the books and talk to the institutions holding the money.