As I just said elsewhere, you wrote the library, so you get to also write the rules... I'm free to wish you would align better with my needs, and I'm able to tell you that you'd have at least one customer willing to pay. Beyond that, well...
An online service would be acceptable because I can still semi-automate that (using curl or wget), and it would still be better than remembering to make sure I've re-exported every single diagram I've updated before regenerating the master PDF.
e.g.
#/bin/sh
# online GraphML-to-X conversion
# usage: yworks-export.sh userid password graphml-filename output-filename output-format
curl --silent --fail --show-error --anyauth --user ${1}:${2} --form file=@${3}\;type=application/octet-stream --output ${5}
https://autoconvert.yworks.com/convert?format=${6}
...and voila, I have something that can be embedded in my document-processing pipeline, and it's something you can track and charge me for. There's an almost infinite number of ways to authenticate a remote user, and at least 3 or 4 of them are reasonably secure :-). My internet connection(s) are sufficiently robust, and my needs sufficiently non-critical, that the occasional downtime isn't a major concern. (Plus, I can always do it by hand from yEd!)
If you ever developed such a service, I'd be willing to pay a monthly or annual fee to use it - somewhere in the neighbourhood of $10-20/month or $100-175/year would probably fly in the general market. My sense is that the people who really want to automate GraphML conversion REALLY REALLY REALLY want to automate it, but only at a moderate price point - $2700 just seems ridiculous unless I'm in the software business.
Perhaps you have a partner company or client who'd be willing to develop such a service, or product as described previously?