The alternative being to provide another platform with some kind of native support in addition to web technologies? Should it be android, qt, winforms etc? I'm not sure this buys you anything in even the short term of the next few years.
Web technologies are moving fast. It is a compelling argument to develop a web only application to gain cross platform support. The feature sets of web technologies is converging on what is possible with native applications and aiming at where things will be in the next few years seems a better use of effort than providing some other mixed development environment. Nevermind that any apps developed for this platform will work with likely minor changes on all of the other platforms.
Directly towards your point, I'm aware of several companies that are rewriting their native applications for iPhone etc to use web technologies, explicitly because they don't want to deal with multiple platforms. It may reduce the feature set is some cases but it seems pretty clear that the web api is getting closer to native apis all of the time.