It's definitely a better representation of Bitcoin's dominance, but what's even better is to drop this entirely.
I completely agree with you, but I just can't see it happening unfortunately.
If we were able to rank coins by some less tangible quality such as "working product", "real world use", "future potential", etc, then we all know that bitcoin would come first by a long, long way. But it is human nature to try to find order, even where none exists, and people like making lists with easy to understand metrics and numbers (such as marketcap), regardless of how useful or not those lists may be.
It will take time before the current get rich quick noobs get there.
Yup, but as soon as we hit the next bull run, there will be a new influx of get rich quickers ready to throw their money away on any and every new useless token.