Leave a good legacy
First of all, please don't just curse legacy, as without it we would get nowhere. Most new inventions in the world nowadays seem to be created by refining or combining existing inventions - breakthroughs are very rare. This almost always the case in Web Development; If something is not applying good concepts from before, they are avoiding bad patterns from the past. The past may not teach you what to do, but what not to do.
The average web developer likely curses legacy a minimum of a hundred times a day. It may be Internet Explorer or old jQuery code written in 2010. Just remember if we had no IE6 we would have no AJAX. If we had no jQuery we would not have millions of developers writing front end application logic. While jQuery itself has partly been rendered useless, the API remains very common and powerful. Without IE6 & jQuery we'd have something different, maybe even something better.
Your code will be legacy
As is gradually rewriting running legacy applications in PHP. It is perfectly feasible and quite a rewarding task in itself. It's practical for businesses, gives you possible to maintain stability and allows improving the developer experience as you go.
JS and PHP are going through the exact same renaissance right now. We should look to each other for inspiration, not shaming. #frontinams
— Rafael Dohms (@rdohms) August 28, 2015
Building a new CMS in Go is a super cool and and a great way to learn, but why not think of a completely new idea and write it in PHP? Many of the basics that you need to solve have been solved with PHP. I think having a unique idea is infinitely harder than the mechanical execution of an existing invention using an alternative tool.
Also it's common to curse the developers of the old systems' developers as flat foreheaded idiots. I would like to believe nobody on purpose writes code that is bad. Limited experience, varying tastes and circumstances do lead to horrible systems - but I can't imagine any motivation to deliberately write horrible code for the sake of it. Most old code is just that: old.
Create a virtuous circle: Build on good legacy, leave good legacy.