Most of my inlines are #11 cap ignition, and they work just fine. As long as you shoot loose powder. I think the 209 came into life because people wanted a hotter ignition source, easier to handle, and more weather proof (although a #11 cap IMO is very weather resistant). Then when they started shooting the 209 in loose powder and shooting pellets they discovered the advantage of the 209 overall for pellets.

When the crud ring issues all started coming to light, almost at once, less powerful 209 primers came out and then different loading systems like the 25 ACP came in. With loose powder in my inlines, I have never had a #11 not do the job.