That wouldn’t explain why some fail in hours when there are tons of connections being opened and closed (like with torrenting) whilst others with only twice or four times the memory will last for months, which is far more that two or four times as long.

I think it’s mainly memory leaks exactly because in my experience the failure rate seems proportional to Free Memory and frequency of memory allocation (hence hitting much more cheaper low memory routers where a much larger proportion of memory is allocated just for static code and data, in usage situations were a ton of small memory allocations are being allocated, than routers with a bit more memory or in normal usage situations) rather than proportional to Total memory in-use even when statically allocated.

Not to say that the problem you describe doesn’t happen, just that it doesn’t really explain the distribution in frequency of failures depending on router memory and router usage pattern.