Could that last bit not work to our advantage, at least in the short to medium term? i.e. what "big club" would want to take a punt on a manager who is likely to be lured away to manage the national team within a relatively short period of time?
That said, if the Netherlands start the world cup qualifiers well, and indeed go on to qualify for the world cup, then it would be at least 2 years before the 'shoe in' would occur.
I think it all points towards Koeman signing 1 year rolling extensions at Saints until such time the Dutch job comes calling. Unless, of course, after being snubbed by the Dutch FA a couple of years ago, Koeman's priorities have changed whereby he would now turn down the chance to manage the national team in preference for managing a 'big club' for the foreseeable future.