My longest lens is currently a 70-200 F4L IS and I am shopping for something longer for quite some time now.
I don't like the Canon 100-400 with its ancient IS and awkward push-pull zoom design, and optics could be better, too. A successor (if it ever comes into being) will likely cost 2,5k€ and is therefore most likely out of the question for a hobbyist.
The Sigma 120-300 2.8 OS has pretty good optics but is not as long as I would like it to be, heavy as hell and quite pricey, too.
The various Sigron 50/120/150/200-400/500 f4-f6.3 with and without OS/VC also do not cut it optically.
So lets hope Tamron delivers on this one.