I also doubt that. Of course streams are making a small difference. But honestly, who is watching the streams? First of all RTL has since this year no more live streaming. Than we have Sky Go, but thats a different audience. Just some hardcore fans who are feed up from RTL or from their commercial breaks are actually making the effort to watch (ilegal) streamings. That is a very small amount in the end.
I have a very different theory about why the audience is getting older (besides of the current state of F1 which is not attracting new fans). When I was a kid, I was already a petrolhead, I couldnt wait anymore to finally make my driver licence. And everyone was so. It was not long time ago, I'm now in my 20s. Todays younger people are different now: more ecologically. When you talk to younger people if they are excited for their driver licence they are replying that traveling by train or bus is also fine - of course exceptions are proving the rule. Its not just that the people are loosing the interest into F1 - every racing series has this problem now. Thats my theory at least - of course I could be wrong.
Horses not being a status transport symbol that everyone aspired to hasn't made horse sports decline - Germany managed to pack Aachen out for the Europeans at the weekend despite nobody travelling there by four hooves.. If the problem is that motorsport is only interesting to people with motors, then maybe it should focus more on the sport. I watched the 100m / F1 / Euros over three screens yesterday, and the likelihood of me buying a 10 million German Sport Horse, a German Sports Car or a pair of running spikes are roughly equal.... (though with the amount that the Mercedes cars were on screen if I do buy a car it's going to be the fault of the German showjumping not the F1... )
The question I would like to ask though, is how much does the quality of the broadcast affect the audience numbers? Germans don't seem massively complimentary about their own programming, but sometimes it's hard to tell the difference between genuine complaints and affectionate eyerolling at the quirks of a generally good broadcast.