The foundation and validators have various ways of making a return

The users do not.

Sure but how is that an investment return? Usually when you hear "take a portion of the investment returns" you're thinking expense ratios, some small fraction of the returns that the user is getting.

If the user gets no additional returns, then it's just a fee, no?

Validators pay $10,000,000 for the privilege of processing transactions and collect transaction fees and can probably resale that privilege with a premium based on their absolute return, which would be called yield. They buy originally from the foundation, and thats how the foundation makes its first $1,000,000,000 dollars. These validators and other accredited investors also get access to a completely parallel asset called LIT, Libra Investment Token, which has value based on typical securities fundamentals.

The users get the stablecoin and are resigned to only possibly trying to speculate on foreign exchange rate changes, which are entirely inferior to the other opportunities such that it shouldn't even be considered. Users only have the opportunity to be users.

Probably time to read the white paper again if you really want to have a more nuanced discussion.

Isn't it highly likely that any crypto currency will be seen as a vehicle for speculation?

Libra is a money market fund with a unique way of tracking ownership. That's it.