Right now, I'm investing in $ISHARES CORE EQUITY ETF PORTFOL...

Right now, I'm investing in $ISHARES CORE EQUITY ETF PORTFOLIO UNITS CAD (XEQT.CA)$ because I want an all-in-one, well diversified equities product with low fees. I'm aiming for retirement in 30-35 years. The low fee aspect is why I've chosen XEQT over something like a traditional mutual fund.
Let's say 30 years from now, we get an all-equity "super ETF" that is to ETFs now, what mutual funds were to ETFs. Let's imagine this super ETF (let's call it SuperXEQT) is also distributed by BlackRock and has the exact same holdings in the exact same ratios as XEQT, but it has a MER of 0.1 instead of 0.2. It's a mathematically superior product.
When I go to cash out of XEQT for my retirement, why would someone buy my XEQT ETFs when they could buy SuperXEQT instead? The only thing I can think of is that I'll have to sell my XEQT at a bigger discount than I would if SuperXEQT didn't exist, which makes me wonder how that would impact my returns in the long run.