Most yeasts have krausen that collapse as fermentation ceases, so are unsuitable for open fermentation for all the reasons feared. If your yeast is like those below, then it could be suitable for open fermentation.
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As above, suitable yeasts sit on top of the wort unless forcibly returned. Left unroused the wort would eventually fully ferment, but with only a small percentage active it would take a week or more rather than a couple of days. So the wort is roused for a few minutes every hour or so until fermentation peaks, when the surplus is harvested, or in the case of Yorkshire Squares, retained in its upper section. The wort is then cooled, the wort clears and the crust stays firmly on top.
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Above is my open fermentation with a similar yeast after harvesting on day 3. The yeast reached the top of the vessel and a covering left for protection when harvested.
The picture below was taken 4 days later, as the beer was racked and the crust only then began to break up. If the beer was chilled or left for another 2 weeks, then potentially the yeast could sink, but those aren't part of open fermentation procedure.
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