I have to wonder about the Clinton campaign's view of Sanders' campaign. It may well be that they are happy about it, and may well support it, as it could be useful for the Clinton campaign.

Unlike Warren, who may have had a slight chance, Sanders has no realistic chance of winning the nomination (see Triseult's comment, for example). At the same time, he does have a small group of very vocal supporters, has some real support behind his views, is running a mild campaign, and is actually a sane candidate. Since they share many policy positions, debates between them could allow Clinton to explain her policy positions, to a receptive audience, more strongly and widely than she would otherwise, and there may be a benefit in being able to agree with some of Sanders' positions rather than bringing them up herself. She may well be able to position herself in debates as the candidate who can take Sanders ideas and realistically implement them, drawing energy and support from the dedicated Sanders supporters without needing to build up that same level of fanatical support, with its drawbacks, for herself. At the same time, the rabid Sanders supporters who are ignoring his calls for a clean campaign and trying to bash Clinton online will make it very likely that many anti-Clinton talking points and potential scandals will already be old news by the time of the real campaign.

There was, if I recall, polling suggesting that a significant number of Democrats wanted Clinton to win the nomination, but didn't want her to do so unchallenged. Sanders allows her to have the benefits of a good challenger with comparatively little risk.