What if the person who championed this leaves?

This is one of the most common ways a perfectly good system quietly dies, and almost nobody plans for it.

Someone drives it. They understand why it exists and they defend it when it wobbles. Then they move on, and within six months the team has drifted back to the spreadsheet, because nobody left knows why the new way is better.

What prevents it

Write down the why, not just the how. The user guide explains which button. The decision record explains why the process changed. When the champion leaves, the second one is what survives.

More than one person who understands it. If only one person can explain the system to a new starter, you have a single point of failure with a notice period.

Make it load-bearing. Systems that sit alongside the old way get abandoned. Systems that the old way now depends on do not. If the spreadsheet still works, people go back to the spreadsheet under pressure.

The uncomfortable version

Sometimes a build was really one person's project rather than the business's. It was their idea, their budget, and their enthusiasm holding it up.

If that is the case, the honest thing is to notice it during discovery rather than after they resign. I would rather ask early who else wants this and get an awkward silence than find out later.