I was trying to work out why what I'd said might be stupid.
It occurred to me that the makers of MMR vaccine and the makers of the single vaccines might be different. It turns out that Merck made both the single mumps vaccine and MMR but not measles or rubella vaccines. So maybe Clayton thinks that Merck had a cunning plan to increase sales of MMR by withdrawing the mumps vaccine but this wouldn't affect the other single vaccines which aren't made by Merck? That's the best explanation I can think of.
Of course, that would assume that they'd actually get an increase in sales of MMR by withdrawing the single mumps vaccine - and an increase that would be greater than the loss of income from the single mumps vaccine. And that they would, like Clayton, be willing to see children who didn't get MMR get sick because they were left unprotected from mumps. So for this to be the case, we'd have to assume that (a) Merck thought there would be a financial advantage and (b) that Merck are as callous as Clayton.
On the other hand, Sanofi Pasteur have single measles and rubella vaccines licensed for use in the UK and an MMR vaccine licensed for use in the UK. They haven't withdrawn their single measles vaccine. Even if Merck did think that withdrawing single vaccines would increase their profit from MMR (and that's a pretty big assumption), it seems that Sanofi Pasteur don't.