Well that’s true, you can move your index fund admiral shares in kind to Fidelity now, but buying more of any Vanguard fund will cost you a $75 transaction fee (not including automatically reinvested dividends), so you would end up buying the Fidelity index fund instead to avoid that fee. Selling a Vanguard fund at Fidelity - there is no transaction fee. So that would probably be the main reason for continuing to hold such funds through Vanguard if you wanted to be able to continue to add to them without transaction fees.
I’ve owned the Vanguard short-term bond index fund for a long time in my Fidelity account. Fidelity had no equivalent. I had to pay $75 for each purchase. I simply did purchases in large amounts such that the transaction fee hit didn’t add that much to the very low ER, at least when compared to my other funds at the time. Eventually, a couple of years ago, Fidelity finally came out with their own short-term bond index fund, so I switched to buying that instead, but still hold the legacy Vanguard equivalent. Lo and behold last year my Vanguard short-term bond index was automatically switched to Admiral shares! A non taxable event. It won’t cost me any transaction fee to sell from my Vanguard fund holdings at Fidelity but I’m in no hurry.