Stats have shown that the vast majority of people use the web interface, the mobile web interface, and Twitter's own clients (in that order). After that comes third-party clients in a small slice of the pie.

Also, non-end-user clients (e.g. Hootsuite) won't get shut out because they provide value above and beyond 'list of tweets', and Tweetdeck is owned by Twitter now so they're going to be fine too.

Twitter could turn off all third-party clients tomorrow and the vast majority of Twitter users wouldn't notice anything (until the minority started complaining loudly). Even then, most people would probably think 'Well, just use one that works then.'