One thing you can be sure. It's very competitive between those telco's.

A. Today, it might be telco A who has the best deal.
B. Tomorrow, it might be telco B.
C. After that, it could be telco C.

If you think you got a good deal now, it is likely to change in future.

It is true that existing customers often do not get offered the new attractive deals offered to new customers, and that tends to happen with most types of companies.

The question is whether it is worth your while to monitor these things, and take the time to work out whether it's better or worse in your case, and switch if not.

I have done it before, but realistically, in the longer term, there are not much difference between the telcos. Why? if they are consistently worse than the competitors, they will go out of business.

So, in the longer term, there's not much difference between them.