Our neighbor has been removing and cleaning up some overgrown shrubs and trees recently.
While the plants added some nice privacy between our yards, a set of invasive blackberries took up residence under them and started to take over their yard (as blackberries will do).
So they've been cutting, digging, and ripping out a lot of the overgrowth.
It's a bit sad but if they didn't, the entire neighborhood would be consumed with blackberry brambles in the next few months.
Oftentimes a bit of pruning and clearing is needed in the yard in order for everything to grow right.
The same can be said of your Shopify store.
Over the years it can accumulate cast-off code, failed attempts, and in-general a lot of extra junk that hurts your performance (sales and speed). In programmer-lingo, that's all "cruft".
The best option is to go through your cruft regularly and remove it before it gets overgrown. This would be trimming branches and pulling weeds when they are small.
But if your theme's code has turned into the Shopify-equivalent of my neighbor's yard, you might need to do what they do and hire someone to help. But instead of a gardener you need to hire a Shopify theme designer who has experience with weeding and pruning a Shopify theme.
Good thing I'm married to one. If you'd like to get your Shopify theme de-crufted and fixed up, Ilana's Website Rescue might be what you need to get back a clean (and converting) Shopify theme.