Flash, he argued, was cumbersome to use on a touchscreen, unreliable, a security threat and a drain on battery life.
He said videos and animations could instead be delivered with HTML5 and other open technologies, making Flash redundant on a smartphone or tablet.
"When the iPhone came out, Flash wasn't quite ready," Mr Mendels told the BBC.
"But also I think Apple wanted to create an Apple-only ecosystem."
Eventually, Adobe did get a version of its Flash Player working on smartphones.
But the internet had moved on. Big brands such as Facebook, Netflix and YouTube were already streaming videos to smartphones without Flash and in November 2011 Adobe ended development of Flash for mobile devices.
It continued to produce Flash for desktop computers, but the software suffered from multiple security flaws.
In 2015, Apple disabled the plug-in in its Safari web browser by default, and Google's Chrome started blocking some pieces of Flash content.
In July 2017, Adobe announced that it would retire Flash in 2020.
It said other technologies, such as HTML5 had matured enough to provide a "viable alternative", without requiring users to install and update a dedicated plug-in.
The Internet Archive is currently hosting more than 2,000 items.
Its collection includes episodes of Salad Fingers, although David Firth has posted official copies on YouTube, which he considers to have been a "Flash killer".
"As time went on and YouTube offered higher and higher-quality video formats, there was simply no reason to post in the Flash format," he explained.
But since Flash was also used for interactive websites and games, there was "every reason to preserve the format", he told the BBC.
Many of the features animators used are still available in Adobe Animate.
In its final update, Adobe said: "We want to take a moment to thank all of our customers and developers who have used and created amazing Flash Player content over the last two decades.
"We are proud that Flash had a crucial role in evolving web content across animation, interactivity, audio, and video."