Agreed.
There seems to be a conflict of objectives in some cases. Quake is
old technology, and that comes with both pros and cons.
The most obvious advantage is that it can, will, and does run on almost anything. It thrives in low-memory situations, doesn't choke on low-powered CPUs, the game data itself is small, fast to install and fast to load. It doesn't try to do anything too complicated so it's reasonably simple to deal with. It's well understood. These are all valid reasons for selecting Quake as a base for a mod.
Disadvantages are that it needs major wrestling and internal surgery to achieve many things outside of it's limits. It's data formats are also limited. Some of the simple things it does, it does them in a fairly roundabout complicated way. There are a
lot of things fundamentally
wrong with the released source code as-is, and many of these have crept into engine ports without being fixed.
Part of wisdom is knowing when you've made a wrong decision, and knowing to back away from it. If somebody is constantly beating against limits of the Quake engine, then the possibility is there that they've just made a bad choice; maybe the Quake engine isn't suitable for their mod. (Having said that, if they get enjoyment from beating against those limits and fixing them, that's a completely different story. But that approach isn't for everyone. I digress.)
So if that 5000 poly model runs poorly in Quake, remember that Quake was never designed to handle 5000 poly models. You'll need to change it's model format, rewrite the loader, rewrite the renderer, and do a whole bunch of QC to support it. That's the reason why modern games can use 5000 poly models with good performance but Quake can't. Things were different in 1996; 3D acceleration didn't exist, memory was low, performance was low, and Quake was designed around those constraints.
It took Valve almost 10 years to get from Quake to Source after all.
So, like I said, if that model (or whatever else) turns out to be a crucial part of your mod, then maybe Quake is the wrong technology for your mod; maybe an alternative choice that actually supports the things you want to do would be better.