I bought 23rd January on the same day I heard there is no windows 10 update for Lumia 730. Is this true?
All of that Is false information.
All the lumias in the current line up is guaranteed to get W10
EXCEPT for when carriers decide not to release an update. Example: The 810 which T-Mobile US has blocked updates for.
HOWEVER, the Preview program has been an easy workaround, you're missing the firmware updates though.