First I'd like to thank RM for starting this debate. Even though I believe (and will show) that his argument is wrong on all counts I think it's important to analyze what is growing into a dominant subgenre in modern hip hop.
I will first deconstruct and negate the Pro's line of argumentation and then provide my own brief timeline/analysis of the formation of "mumble rap".
Pro's first argument relates to the monikers that some so-called mumble rappers have taken up. Examples include lil' yachty, lil' uzi, and lil' xan. He then proceeds to ponder how these prefixes could possibly be a coincidence before settling on their names as necessarily being tributes to lil Wayne as the originator of their preferred style.
This argument might be convincing if lil Wayne was the one to start the trend of using a "lil" at the beginning of his stage name. Unfortunately for Pro this couldn't be farther from the truth. Cursory research reveals the commonality of this moniker starting in the 80's. The first known rapper to employ the name was Lil Troy (known for his song "Wanna Be a Baller").
Other well known rappers to employ the name include Lil Kim, Lil Jon, Lil Dirk, Lil B, Lil Boosie, and Lil Reese. These rappers came up from all over the country, heralding from Atlanta to Chicago. None of these artists are so-called mumble rappers and a few predate Carter's entry into hip hop entirely. The name has been employed so popularly it even inspired the parody rapper Lil dicky to don the name in 2013 before the rise of the SoundCloud generation.
Pro fails to account for the popularity of the lil moniker before the entrance of lil Wayne to the hip hop stage. It's not lil Wayne, it just happens to be an immensely popular prefix like "Young"(Yung) or MC.
[Flow/Delivery over Lyrics]
Pro's next argument contends that Lil Wayne was the progenitor of a hip hop style which prefers the style of delivery and aesthetic flow to specific lyrical content. This (he claims) is what mumble rap uniquely is and therefore Lil Wayne invented it. He claims that Lil Wayne invented a style of rap which prioritizes talking about money and drugs to more substantive subject matter.
Again, these stylistics pre-date Wayne and are not either unique to so-called mumble rap or what defines it. Going back to Lil troy, his most famous single "Wanna Be a Baller" definitely pre-dates Wayne's entry into hip hop and employs only lyrics of fame and drug consumption. According to Pro's line of argument I suppose it was actually Lil Troy who invented mumble rap.
Although I don't find it necessary see Snoop Dogg, P Diddy, Dr. Dre, Xzibit, etc. etc. as rappers who predate Wayne that dealt with the aforementioned subject matter.
As to the argument prioritizing style to substance, this has never been what strictly defines mumble rap. Since the inception of hip hop there have been artists who prioritized one over the other. even restrictions myself to the "Lil" moniker, on Pro's argumentation it would seem that Lil Jon must be one of the creative forefathers of mumble rap.
Pro also makes reference to some songs predating the rise of mumbe rap where Wayne employs a style of delivery that is difficult to decipher (he might be said to be "mumbling"). I won't dwell on this point too much. I'll simply reference that specific trait in artists pre-dating Wayne: Das EFX's "They Want EFX" and FU-Schnicken's "True Fuschnick" all more resemble modern mumble rap techniques than the Wayne examples Pro provided and were released in the mid-1990's. The latter also employs the use of onomatopea "sound effects" at the end of various lines, predating another "mumble rap" trait found in Migos, 21Savage, and Desiigner.
To summarize my deconstruction of the Pro argument, Lil Wayne was not the originator of any of the individual elements Pro believes consistitute so-called mumble rap. Whether that be the "lil" moniker, the prioritization of style over substance (or delivery over lyrics), money and fame as lyrical inspiration, or even (perhaps the most well known mumble rap trait) onomatopean outbursts.
[What is Mumble Rap]
To define mumble rap we must define what is unique to it. As a subgenre it exists in a specific time with specific attributes which we must be careful to define lest we cross into generalization and definitions ambiguity.
I forward a simple definition of mumble rap (beginning in the 2010's) which employs lo-fi production, trap influences, emphasis on personal aesthetics, and an evolving lyrical style which either employs onomatopean outbursts or highly emotional and vulnerable language. Two classes can be descended from within the genre depending on which of the last two lyrical traits are most present with Lil Uzi, XXXTentacion, and Lil Xan employing the latter and 21Savage and Migos embodying the former.
As a genre "mumble rap" has roots in varying hip hop genres. No single artist originated it but a multitude of trends and styles meshed at a specific to give rise to the genre. The ascension of trap and Gucci Mane's prolific output along with the DIY nature of Chief Keef's production and aesthetic were major influences but the late 2000's alternative rock scene also played a part in the development of some lyrical content. These trends coalesced (initially) around the SoundCloud community in the 2010's with artists like Ski Mask the Slump God, XXXTentacion, Lil Uzi Vert, Rich the Kid, and Lil Yachty.
To Conclude, Lil Wayne did not begin the mumble rap movement anymore than Lil Troy or Lil Jon did. Mumble rap (more aptly called SoundCloud rap) is a genre defined by more than just mumbling or lack of lyrical depth. Hip hop has always contained artists off that caliber. What sets it apart are the specific and unique influences it channels (Mane, Keef, alt rock, DAS EFX) and the community those influences had a major impact and productive output (Soundcloud).