I have Homeseer 2 and lots of X-10 devices.
I bought an ISY-26 because I wanted to operate independent of my PC and via web browser.
From my reading on the web, Insteon and ISY support X-10 (and is also much better, etc.)
From what I can read in this forum the ISY support for X-10 is minimal.
If I had been able to read this dialog last week I would not have ordered an ISY.
I am not about to replace all my X-10s although I would expect to add Insteons over time.
If ISY was my product I would support older vintage technology as a way to attract people like me and migrate them to Insteon. Even Microsoft supports older versions.
The tab where you add X-10 devices sounds like enough.
If you are not going to do this, I would ask you to make clear to buyers that ISY really doesn't support X-10.