1.) Why did Canon raise the price of other lenses too? 500,600,800... are about 500-800$ more expensive overnight.
2.) Are there alternatives for the 500,... lenses? Sigma?
Half of the price. How good are they?
1.) Not as far as I can tell. The 300 - 600 II lenses haven't seen a price change in several weeks, and they're a couple to a few hundred cheaper than at launch. They did all see a big increase in between announcement and launch - but that was a 12-18 month interval, and IIRC there were currency issues then.
2.) The Sigma 120-300/2.8 is reportedly very good, although not quite the IQ of the 300 II. But longer offerings from 3rd parties don't come close to the Canon supertele lenses.
Ad 1.) Not in Germany..... (Picture from 600mmm Canon L II)