Will some of your new readers be disappointed when your oneshot ends and wander away, looking for the work of other creators to read? Yes, most likely.
But some of them will stay, and look forward to what else you create. From a reader's perspective, I would much rather see a creator who makes worthwhile things, regardless of length, than someone who looks first at how successful a format would be in terms of readership before choosing what to post.
From a creator's perspective, I think short stories/oneshots are a powerful format with a lot of potential, and would love to do more of them. There are benefits to longer comics - enough space for all the stuff you want to accomplish, room to build complex storylines and explore a world, etc,. etc. - but there are strengths in the shorter format as well. The limitation in length forces me as a creator to be clear and get to the point, which leads to practising good pacing and clarity. I can then take those skills and apply them to longer stories, which is all good.
I'm currently running both a longer story and a short winter-themed oneshot for December - the latter is called Wishlight, and I really, really enjoyed making it. <3 It was a nice change of pace from my longer comic, and let me practise stuff like panel-composition and allowed me to experiment with stuff.