Argument
Keanu ruined all the movies he was in
Rebuttal
This is often an example of confirmation bias. Speed 2 did horribly; if Keanu had been on it, there's a very good chance people would have heaped much of the blame on him. Yet the movie still tanked despite (because of?) his absence.
Many of his movies instead benefited from his presence, where some would have never made it off the ground had Keanu not been there. On The Replacements and The Devil's Advocate, the studios could only afford to hire Gene Hackman and Al Pacino respectively because Keanu deferred a large part of his salary – 90% of it, in the former case. On A Scanner Darkly, Robert Downey Jr. only agreed to come on because Keanu was already part of the cast. Like Thumbsucker, it only managed to get green-lit by the studios because of the big names present.
On Speed, Keanu made many (uncredited) major changes to the script which primarily involved turning Jack Traven into an earnest guy who really wanted to help people, instead of his original incarnation as a macho dude blowing things up and shooting off witty one-liners. The movie could have been very different.
Probably the best example of Keanu contributing to a film's success is The Matrix; the whole project might have been dropped without him. He was the only actor around who understood the script and was willing to commit to all the pre-shoot preparation that the Wachowskis demanded – such as finishing off a reading list of books before even opening the script, and then spending three months on a punishing training schedule that regularly left him bruised all over. He believed in the movie enough to channel some of his own money towards it when the Wachowskis were running short on cash and thinking about giving up. They had nothing but praise for Keanu's dedication to the film, citing him as an inspiration to everyone. (With thanks to ckage.)