The company has been cautious when adding new currencies to the exchange, and is working to avoid any "rubbish out there", Hirji said.
They say they're actively working to avoid adding "rubbish" currencies but I think some might argue bitcoin cash is one of the biggest "rubbish" currencies out there adding little value to bitcoin or crypto in while diluting/diminishing their net worth. There could be a respectable amount of politics in play here. Coinbase could utilize their high market share and influence towards legitimizing or delegitimizing certain altcoins via whether or not they're listed on coinbase's index. This index could be utilized as a means of controlling which altcoins are legitimate.
I doubt this will do anything to make progress on a bitcoin ETF. Any bitcoin or crypto website which provides financial charts could probably create their own crypto indexes. There is no set standard and coinbase's index won't necessarily be any more credible or legitimate than anyone else's. The only reason coinbase has a chance of legitimizing an index involves them having a large percentage of market share and the largest userbase but it is possible that will come to an end if japan and other nations get involved, which could suggest whatever index coinbase creates will be short lived.
Also coinbase could lose market share over time as users flock to other exchanges which offer a wider selection of crypto as well as in the case of exchanges like robinhood, lower trade commissions.