Introducing HTTPS to existing infrastructures is easy. Just terminate it at some point. It also adds a big advantage to users that their traffic can't be read by network operators. Introducing ipv6 is harder, as you need to change all your routing, logging, banning, etc. facilities to support ipv6. It doesn't add any direct advantages for users, as all ISPs still have to support ipv4.

> Lets Encrypt was released to the public in 2014 and by like... 2019 basically all of the internet was HTTPS

Google's ranking bonus had also been a great incentive.

> Lets Encrypt was released to the public in 2014 and by like... 2019 basically all of the internet was HTTPS.

This is apples and oranges: absolutely zero software upgrades needed to be done to get HTTPS going and/or Let's Encrypt running.

I was able to get LE going on our F5 appliances in a few working days with zero changes to the base system/appliance software by simply installing the dehydrated ACME client and all of a sudden dozens of sites where we previously didn't want to pay for a cert were "secure".

Network hardware can stay in place for quite a while. Our previous generation of core switches lasted us 7 years before we swapped them out.

I wouldn't be surprised some of the mega-chassis routers in ISPs and other telcos sit around as long.

7 years for a core router is on the low end aswell. High end routers consist of a chassis which can last a decade or more easily. usually the line cards inside the chassis are replaced to allow higher band with, but the control plane can stay in place for a very long time.