I don't believe Sarah will will steal the spotlight from the monarch and heirs per se. However, if history is anything to go by, there is a reasonable chance that she will do or say something wholly inappropriate, and potentially bring disrepute to the BRF. The Firm is suitably insulated as long as she is the long divorced wife of Andrew, a position they will lose if she were to be HRH The Duchess of York once again.
With respect, I think this is just some nonsense thought up by the tabloids to keep the clicks coming.
I think that makes sense, and if they were to remarry, it may well be in the next reign.
My bigger point is that I am no tsure marriage will bring that much more to Sarah and Andrew than the current position. They appear to be enjoying the benefits of being a couple, without the responsibilities that may come with them being married, and that must be a good thing from their perpsective.