Looking at the Wikipedia article on basic reproduction number, it looks like the most contagious virus (as of right now) is measles with an R₀ (high estimate) of 18. I'm wondering if there is some asymptotic limit to how contagious viruses can get, and maybe measles is close there? If there is or isn't, do we have any ideas as to what biological mechanisms are related to this? 

I'm asking because it seems as the new variants emerge, it would be wise to aware of the worst case scenario. Just making a rough plot of the Covid rows in the basic reproduction article, and a rough stab on when they emerged, the wild-type, Alpha, Delta trend looks like this. 

From Tomas Pueyo's excellent thread, it looks like as Covid gets more transmissible it's likely to get more deadly.

In 20 months Covid has went from an R₀ (again, high estimate) of 3.4 to 8. I'm not an epidemiologist, but that seems like a really big jump in context. To be fair, again, I'm using the high estimates, but the general trend is concerning and I would sleep better knowing if there was some biological reason it would plateau.  

A scary, but I'm not sure how likely, scenario would be if another 20 months out (March 2023 or so), assuming there's large enough sections of the world where Covid is still spreading (either because vaccines haven't gotten there, or new vaccines are required for breakthroughs, or vaccines/infection immunity declines too quickly, etc), is there any reason to think it wouldn't jump another ~4.6 to ~12 or higher? Could it go above 18? 

 I'm just surprised I'm not seeing more discussion about this. Maybe I've missed it?