It's tough to see a future for 500, which is written in a language that is no longer supported. Rewriting in something else would be hugely expensive, and it's hard to see how that investment would not pay off more and faster if applied to X3 functionality.
IMO opinion we will not get any direct guidance on this, because I doubt UK wants to repeat its boneheaded replay of announcing that they were selling off a product before they had a buyer in hand. I don't know where 100 will be in 10 yrs, but I suspect that Sage will attempt to up its game a bit for enhancements there, so that it can keep its M&S flow going as long as possible.
As a partner, it means we really should be making the effort to help our customers understand the financial costs and benefits of staying or leaving, and if leaving, then understanding why and when.