If you are the internal IT person reading this, you already know how this normally goes. A managed provider gets a foot in the door on something narrow, spends a year finding fault with how things are run, and eventually the conversation with your director is about whether they need you at all.
That is not the arrangement, and it would be a poor business model anyway. Co-managed clients stay for years precisely because their internal person makes the relationship work.
In practice we usually make the internal person’s life better, because the tickets they hate are the security alerts they cannot interpret and the vulnerability reports nobody has time to work through. Those become ours. What they keep is the part they are actually good at, which is knowing the business and the people in it, and we are not going to be better at that than somebody who sits there every day.
It also works the other way round. Your IT person gets somebody to think out loud with, a second opinion on a decision at four in the afternoon, and cover when they are on holiday or off sick, which for a team of one is usually the real problem.
You should be able to go away for a fortnight without the business being less secure than it was when you left.