as mentioned above, my P30 had a 7/8 shaft, my friends Cat30 has a 1" shaft... even C and C 27's with A4 have 7/8 shafts, and those companies also have engineers. Ericsons are generally well designed and well built boats, but they were not immune from the occasional cost cutting shortcuts. The rudders on the 35ll were one notable example, using the rudder from the 32 apparently... which is at least one reason why these boats are virtually uncontrollable in reverse. Also, the mounting of hardware, chainplates etc through cored deck which has caused such huge issues later on.
So if not under-spec'd, 3/4 shafts might be at least marginally spec'd
There is no yard time, the boat is in the yard; there is no labour cost, I will do it myself; the cutless bearing is not perfectly good as it requires replacement anyway; the strut is perfectly good and will remain, the new cutless bearing will have the same OD but 7/8 ID; and the propeller is the 7/8 Indigo that I want to put into use. I simply need a new shaft for a couple hundred dollars, which to me is money well spent.
''Still working so leave it'' is rarely a good reason to not do something. My chainplates look fine and are still working, but are 40 years old and I will replace them. My stays are still fine, but 40 years old so I will replace them. The same logic can be applied to a marginally spec'd 40 year old SS shaft. One does not wait for any of these things to break before one replaces them because, if they do go, they can ruin your entire day.
This is simply another 'your boat, your choice' kind of thing.