If the original reasons for leaving haven't changed, leaving in 6, 9 or however many months might mean taking a pay cut.
In the case of the what was being discussed in that branch of the conversation, it was explicitly stated "if it's
only money", i.e. the original reason for leaving is because of money. It is also why I opened with "All else being equal", i.e. the other conditions haven't deteriorated during that period, which is plausible given we are stating in this scenario that they are "moving on shortly after".
To flesh out the example it could be something like:
-Current package from Company A = £30k. Looking to leave because you want more money.
-Company B offers £33k
-Company A counter offers at £38k
-Counter offer accepted
-Shortly after you leave company A for company C. Given your reasons for leaving company A are already defined as only about money, one can infer that the reason you have have joined company C is because they are paying you more money. So you've got a triple-helping of pay rises compared to the single-helping that was originally on offer from company B
It also depends how you're finding jobs and offers, if you're in the sort of role that tend to go through agencies and you turn a couple of offers down you'll soon get known for it.
I had one person turn down an offer in order to stay where they were, I wouldn't waste my time interviewing them again should they ever apply.
This is a fair comment, however I can't imagine you are going to be turning down that many offers, given in this example we've already stated that you are going to be moving on shortly. Or to play devils advocate, if you've turned down that many offers that you've got widely black-listed, you're probably minting it anyway from all those counter-offers you've trousered (remember, in this hypothetical scenario the reason for leaving is only about money). Sure, one day in years to come you might be looking to move on and be motivated by other factors, but unless you work in a very niche sector I doubt you will get frozen out indefinitely, especially given the way agents themselves move around all the time.