Op
i hope you are right, and that they are just trying to formalise a routine pattern.
But, and I mean this in a helpful way, not a spiteful one- in your shoes I would be concerned. As I said, vanishingly few people are utterly irreplaceable, don’t kid yourself that you are. They might prefer to keep you than lose you, but if you really can’t fulfil the hours they believe they need, they might decide to move on.
Secondly, if I have understood the situation accurately you manage the team/department. Yet without you there, in person, to direct (and thus, arguably, micromanage) them things fall apart to such a degree that your seniors are concerned they would not manage to honour/deliver a major contract. You being key to that contract being delivered last time is all well and good- yes, that’s a plus. However, having recognised how bad it was, you as the manager of the struggling department have not made changes/put things in place/ upskilled your team and so on, in order to address the weaknesses/skill gaps/organisational flaws that you (and others) identified within the team you manage. If those changes were not within your scope to make, did you escalate this (and if so, can you prove you did)? Now, you remain in the position that you are not confident that your team can meet the challenge of fulfilling thst contract again (despite knowing it was a company aim to win the contract)- and what’s more, your senior management seem to be aware of your lack of confidence in your team. Worse, they share your concerns. If that is an accurate reflection of the situation, I’m not sure you are in quite as strong a negotiating position as you believe when you add in the situation of wanting your hours changed.
Maybe I am misunderstanding your position in terms of the management structure- but if you are the head of a team with problems, or which only works when you are there to micromanage them, ultimately the buck stops with you.