Ask yourself what happens if you kill all those redirects. Ask yourself what technical debt you're creating for yourself with all those redirects.
To me, there are rational scenarios where covering bases with redirects makes total sense, but I don't see a lot of business rationale behind managing THIS MANY redirects. With the tech risks, and the level of effort.
If it were my site, I'd likely (obviously there are caveats) do the following:
1. Delete all redirects that are more than 1 year old. Yes, all. There are likely thousands there that some business person would say, "But we need that!" and you do not, in fact, need that.
2. Audit remaining redirects for business need. Do you need a 1:1 redirect? Do they REALLY need it?
3. If the answer is "yes, the user absolutely needs to end up on a specific page with no extra steps" then redirect. Examples of this might be customer service, pages attached to legal compliance, etc.
4. A lot of the time, the answer will be no. This is ESPECIALLY true in ecommerce with high inventory turnover and product seasonality. In these cases, a high quality 404 with user-friendly next steps (search bar, secondary menu options, etc.) captured most potential.