Actually, they can be used for a broad variety of partner flights as well. However, nearly all* of those partners are overseas.

Miles depreciate, but nobody can say at what rate. It is possible, but highly unlikely, that they would become valueless; the situation would have to get much, much, much worse than it is today, because UA has borrowed against the value of the MileagePlus program. If UA were to go bankrupt, the creditors whose loans are secured by MileagePlus would try to preserve the value of the program, likely by selling it to a competitor. Even in some of the more famous US airline bankruptcies of the past 40 years (in the frequent-flyer era), the programs have been preserved: PanAm's program was purchased by Delta, for example.

UA and Chase are desperate to hit spending targets on the Explorer card. Assuming that you're buying those groceries / gas anyway, and you're not carrying a balance (and thus not paying any interest), this could be a potentially useful way to increase your mileage total a bit. There is a little-known program called MileagePlus Choices that can be used to redeem miles that have been earned from a Chase credit card to receive a statement credit on a future United purchase: https://www.flyertalk.com/forum/unit...5-forward.html . So, this provides a value floor: if you earn, say, 4000 miles from buying $800 worth of groceries, and you didn't have any other use for the miles, you could redeem them for a $40 statement credit against a future United flight. It won't help with the 180,000 miles that are in your account, though; those can only be used for flights, or for merchandise at really terrible rates.

* They have partnerships with Boutique Air, Cape Air, Silver Airways, and Hawaiian Airlines (for inter-island travel only), but those airlines have pretty limited route networks, and award availability isn't always great.