FWIW you are the first person to come here with that problem on a Simplicity.
The wheel would have been cambers in for quite some time and that crack has taken 19 years to fail.
Never replaced one on a simplicity but done a few JD's & Huskies.
Worst case would be to buy 2 new assemblies and put one aside just in case.
Not many mowers now days go 10 years let alone 20.
"First person..." While that may be true on this forum, it's not on another. In any case, I usually apply the following logic to evaluating postings on the web: For every post I see I expect there are at least 500 similar stories out there that are not posted. People generally just deal with mechanical failures by paying a pro at a stealer and never go online, while people posting are skewed toward DIY. Even the tiny local Simplicity dealer I visited the day of my breakdown had seen it happen before.
Given the number of tractors in good shape that I see for sale that are well over 20 years old, I think tractors/mowers last a long time. Decks, motors and transmissions naturally die first, but wheel spindles should be the last to go as they are just chunks of metal that should not fail under normal use. There are lots of junkyard finds and parts tractors out there with nothing more left than frames and wheels.
BTW, I reached out to Simplicity just for grins and got back a pithy response about no goodwill being available for a machine of this age. No kidding, LOL
I didn't ask for goodwill, I asked if they had changed their manufacturing process to properly weld and heat treat these parts. Naturally I'd have to get to someone deep in production for that answer and the poor customer service rep likely doesn't have that access.