That is a lot of hydrostatic pressure. That is the problem with tall, cylindroconical fermentation vessels. They place significant hydrostatic pressure on what is in the cone. People who dive experience hydrostatic pressure. If one examines the breweries known for repitching, volume is increased not by growing fermentation vessels up, but by growing them outward.
So as home brewers we are causing issues with tall conical type ferment vessels?
I have seen large, flat, shallow, open air ferment tanks in a few breweries in the Old World.
As Denny mentioned, our volumes are not large enough to exert much in the way of hydrostatic pressure. If temperature is maintained, the average amateur brewers places little in the way of environment stress on a yeast culture, which is why we should be able to serially repitch bottom-cropped yeast if care is made to ensure that average cell age remains young. The major problem with bottom-cropped yeast is wild microflora ingress. Top-cropped yeast is much cleaner in terms of wild microflora because wild microflora does not floc to the top. However, few brewing cultures are true top-croppers these days due to the extensive use of cylindroconical fermentation vessels and I have yet to see a Sacharomyces pastorianus culture floc to the top.
That being said, I am currently reading "Lager" by Dave Carpenter. The book is beautifully put together, but sadly, it has erroneous information with respect to top-cropping. The author states that top-cropping involves harvesting yeast from krausen and that the practice leads yeast that has lost its ability to flocculate, which is about as incorrect as one can get. First off, the rocky brown head that forms first is skimmed. There is very little in the way of re-usable yeast in the brown head. However, the second head that forms it almost pure yeast that has flocculated. A lot of brewers conflate flocculation with sedimentation. Even non-flocculent yeast strains will eventually sediment if given enough time. Flocculation is the aggregation of yeast cells into flocs. Most brewing cultures fall into the NewFlo flocculation phenotype. Flocculaton occurs in NewFlo cultures after the sugars mannose, glucose, sucrose, maltose, and all of the polysaccharides that can be reduced to one of these sugars have been exhausted, which is why spinning a yeast culture to keep it in suspension demonstrates a fundamental lack of understanding of brewing yeast culture biology. Flocculation is inhibited as long as mannose is in solution because mannose binds to the protein-saccharide receptor that is involved in flocculation. We can think of this receptor as the yeast cell wall equivalent of Velcro. What differentiates true top-cropping brewing cultures from the rest is that they entrap CO2
gas when they floc, which is which why they rise to the surface. What keeps a top-cropped culture from becoming powdery (non-flocculent) is the practice of taking the middle head. If we discarded the middle head and kept the last head, our culture would become progressively more powdery.