I personally wouldn't purchase any mercenary runs but I'm not surprised that there are others who would do it. If it's their prerogative and how they wish to play the game, I don't see any issue with that. There are players who want a piece of gear to glamour, a mount, or even a title but don't have the time to dedicate to PuG'ing or statics - so the option to buy these pieces of content is useful for them. There's obviously a market, that market has been around since the beginning of the game when Titan HM runs were sold for relic reaction, and those runs will continue to be sold until this MMO dies.

In terms of equating RMT to clear seller / buying - I don't doubt that there are some people who buy Gil to purchase these clears but broadly generalizing all clear buyers as RMT purchasers shows a level of ignorance that is rather unbecoming. I know for a fact I made enough Gil to buy a Medium house in the 2.X era within the spam of two months due to retainer ventures, desynthing, and converting crafting / gathering materia while doing Beastman dailies. If I can casually generate that much Gil in 8 weeks with minimal effort, any serious Gil maker can do that in probably 1-2 weeks without using RMT.