Those who make it through the funnel with only X are likely more than the X+Y scenario simply because it's less learning and new concepts so drop out will be reduced. This cohort are probably more likely to go on to learn Y if we further assume it was so valuable as to have been "required". In reality requiring X+Y backfires as step 1 because X is all they want and X may be worth 10Y to the student at step 1. The value of Y may increase to >X after learning X.

When I learned basic HTML, CSS and JS. On step 1, JS had essentially no value. CSS sounded like it had value but was a bit nebulous and optional sounding. HTML seemed like an obvious requirement for building a website, so that's where I started. Once I learned HTML, the value of CSS became obvious and I pursued it. JS still seemed esoteric but later once I got HTML/CSS down and wanted to do more JS kept coming up as the solution. So I learned it too. Now, I'd say JS is way more valuable to me than HTML/CSS even though I learned it last.

This was in the 90s BTW and the path wasn't very clear back then. The first book I bought was actually a Perl book and quickly realized it was too advanced of a place for me to start. I learned about Perl after noticing the /cgi-bin/ portion of URLs and wondering what that meant. It had to be something most sites where doing similarly. This was before Google so whatever random SE I used back then told me cgi-bin was associated with Perl scripting for web development. I continued to struggle with the backend stuff (JS was FE only back then) until PHP3 came around. I'm self taught if that wasn't clear.