To be clear, the self-signature on a root certificate is meaningless (it only exists so that root certificates can be parsed with the same code as intermediate and leaf certificates); using SHA-1 there is not a risk.
Let's Encrypt was only founded in 2014, and left beta in April 2016; they just used that cert to start with IIRC. So it's been about 5 years for LE users, not 20.
What criteria would you use to decide expiration dates for 20 years in the future?
[1]: https://crt.sh/?id=8395