There is no incentive for anyone to pay for a transaction other than to move XMR.
I believe most of the transaction volume growth is organic, however, this statement is not strictly true. The privacy features of XMR provide incentive for attackers to transact in an attempt to weaken the privacy. Lets not forget the IRS bounty on tracking Monero.
Gavin visited the CIA in 2011. The Snowden leaks revealed the NSA was tracking bitcoin users as early as 2013. In 2018 the department of defence ran war game scenarios involving a Bitcoin rebellion. If the public even becomes aware of these initiatives it is typically years later.
Rest assured the public IRS announcement is not the only well resourced adversary trying to unravel Monero privacy. They are guaranteed to be running nodes, making transactions, monitoring the networking layer, collecting metadata, and tracking outputs.
This is a tremendous point. And correct. And oversight on my part. But still bullish really... because