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Is that true about dtube videos disappearing after some time?

If you use them for hosting then yes, your videos are replaced by newer ones uploaded to the network after a year, not weeks. But you can use other centralized service providers like youtube, twitch, dailymotion etc. for hosting as well.

Do they stay on the blockchain?

No, Steem only stores text on the chain. Dtube uses IPFS to keep videos decentralized.

Well that doesn't sound like what many are looking for...