This is very good news, though they don't really show their work on that five year number. Are they waiting on a specific operating system to drop below a certain level?
Reading between the lines a bit: the last major vendor to accept the ISRG Root was Microsoft. Windows is supposed to pick up new roots through updates, but there are probably enough systems out there with updates disabled or broken that it's safest to wait a few years for them to cycle out.
I don't have an explanation for the five years beyond your guess.
Rather sooner, at the end of September 2021 the DST Root CA X3 that cross-signs their existing intermediates expires.
In practice many systems don't directly obey expiries baked into root certs, a self-signed root certificate is largely a vehicle for conveniently moving the key inside it, it's not signed by anybody we trust independently so why care what it does or does not say about that key?
And of course if the IdenTrust / ISRG relationship remains good there's no reason IdenTrust can't sign new Let's Encrypt intermediates with another of their CA roots that hasn't expired. The short lifetime of Let's Encrypt leaf certs means they wouldn't even need to have decided before 2021 what to do about this.
They'll probably re-evaluate market share once they get closer to the five year number. I bet they'll find they'll need to cross-sign for more like 10 years.
