Which is not to say that whatever replaces it is guaranteed to be better, but it will at least have a good start by using the right tools for the job.

What happens when this person leaves? What if business starts booming and you need to scale up? Are you really better off sharing a spreadsheet between a 100 or more people?

Just gonna focus on this part here. The last offices I worked in (and the last places I even dealt with tech stuff beyond teaching it to primary-schoolers), "Track Changes" was simply a godsend. One person "owned" the spreadsheet and could allow use or not; and we had it hosted on OneDrive - of course we all had the OneDrive client on our work PCs and it could tie into Excel natively, so we were all using the same document.

If the entirety of a large org needs the same spreadsheet, you're probably looking at a scenario where they really need multiple sheets, but haven't figured out how to do references to other files and sheets yet. But using Track Changes and having one person designated as Owner so that that one individual makes a decision to approve/disapprove all potential changes makes it much easier to let multiple people work together.

> What happens when this person leaves?

That's a management / succession-planning issue. Ad-hoc solutions shouldn't be kept secret, and allowing employees to just go off and do as they please in that regard is a major issue. If all work-related documents are required to be stored in the company cloud, and you storing a new doc alerts your manager, etc., you'll be able to deal with this much easier. One place I worked for a few weeks on a project did a reset on employee's terminals every Friday at around 7PM, and anyone who claimed they lost data or progress from that on Monday was in trouble, because they didn't save company-related data to the document & data server (which obviously did not get reset) - meaning they'd done something against company policy that they'd been told about the day they were hired.