As I understand it, the main things that kill lead-acid cells (in normal service) are:
1) letting a cell stay less than fully charged for more than a couple days, either by the whole battery being discharged and not recharged within a day or two, or not keeping it equalized, so some cell(s) are not fully charged when the charging system decides the battery is full.
2) letting the plates become uncovered by water loss, then be damaged by exposure to air
3) letting the suphuric acid coencentration become different at different heights, due to repeated small discharge-charge cycles with nothing to mix it.
4) Discharging the the battery so deeply that some of the plate material flakes off and piles up in the bottom of the cells - eventually piling up enough to put a high (then progressively lower) resistance leakage across the bottom of the plates, discharging the cell progressively faster (and making it flake faster) until it doesn't stay charged overnight. (One of the advantages of the Sears Die-Hard battery, at least historically, is that it had an extra-big pocket at the bottom to collect these flakes. So it could last longer in the field before they piled up enough to start shorting and self-destructing the cells.
Other mechanisms are much less likely to occur or take a lot longer (such as expansion of the positive pole piece until it starts to crack the case - which takes a decade or more.)
1) is sulfation: As the battery is discharged the sulphate ions from the the plates turn into lead sulfate. At first this is amorphous and easy to turn the battery back into Lead, Lead oxide, and CONCENTRATED sulphuric acid by recharging. But with time (like a couple days) the the lead sulfate gradually crystalizes into, first a form that MIGHT be broken up by specialized charging regimes, then into one that just isn't going to react. The crystals not only stop participating in the reactions, lowering the battery capacity, but they also are highly resistive, tending to form a coating over the plate surfaces, and they're a different density than the other states of the plate, tending to break it up. You can accumulate damage from sulfation in only a few days of undercharge - either overall undercharge (damaging the whole battery) or unequalized operation (damaging the particular cells that had slightly higher self-discharge and thus ended up with persistent lower charge levels).
2) (loss of water from the electrolyte) can occur due to evaporation, but occurs mainly due to overcharge (including the deliberate slight overcharge during equalization), resulting in electrolysis of the water into hydrogen and oxygen. This gas bubbles out of wet cells, which must be refilled with water occasionally to replace it, before the top of the plates are exposed and the cell damaged. Catalytic caps can recombine the bulk of the gas back into water if it isn't generated too rapidly (as can reactions with other components added to the plates in sealed cell designs), greatly extending the time between refills for wet cells (or time until death by dry-out of sealed cells).
3) is stratification. A lot of small discharge-charge cycles can cause this to build up faster than diffusion equalizes it. The differing concentration of acid at different vertical positions results in corrosion of the upper portion of the plates and sulfation of the lower. (You can think of it as the cell being overcharged at the top and undercharged at the bottom.) In wet cells this can be combated by stirring the electrolyte (impractical), shaking the cells to slosh the electrolyte around (happens in vehicular applications), or mixing it using the gas bubbles from slight overcharges or equalizing charges. (A similar effect to stratification can occur if the battery has a substantial temperature gradient between the top and the bottom. Avoiding this is part of the theory behind battery racks that provide a gap, or insulation (such as a layer of wood) between the bottom of the batteries and a concrete floor.)
So if wet-cell batteries are floated to keep them charged or recharged soon after discharge, given regular small equalizing charges, and kept filed by regular maintenance or catalytic caps and occasional maintenance, the short-term failure mechanisms are avoided. Early recharge and regular equalization heads off sulfation from undercharge. Bubbling from the equalization avoids sulfation and oxidation fro stratification. Heading off sulfation and avoiding deep discharges also avoids flaking and the resulting self-destruction. Catalytic caps can make the cells stay wet for years, rather than months, between refills, despite gassing from the equalization/destratification charging regime. So such cells can last a decade or more before the next big mechanism - positive terminal corrosion - takes them down.
But if they're abused, like by deep discharges, being left discharged, or allowed to become unequalized, they can be killed in a matter of weeks, or even days.