To see whether a WT fare is eligible for MFU into WT+ (assuming that there is WT+ award availability), click on the flight number of the flights you're looking at. In most cases, the pop-up window will have a line in the table called "Selling class". Only classes Y, B and H can be MFU'd.

If there is WT+ award availability, you'll usually find T class availability for a WT+ cash ticket. In very many cases, if you satisfy the advance purchase and other restrictions of the T class fare, the price of the T class WT+ ticket is less than the price of a Y, B or H class WT ticket - and you're in WT+ already and don't have to use miles to upgrade.

There may be differences in flexibility, etc, but this is fundamentally why upgrading from WT to WT+ is generally not thought to be good value.