I would nearly garantee the start problem is not in the button - it would be in the relay for the statrer (RH side cover).
What hapends is over time the crappy thin terminals inside the relay mounting block weeken and loose grip therfore giving limited to no contact.
I suggest replacing ALL the terminals in the relay mounting block with better ones - do this one at a time so you get all terminals bach in the right place in the relay block.
A long time ago i modified the terminals to eliminate the power from going thru the ign to starter
Now it goes from bat to relay.i
I think i have saved my ign but also saved the terminals/relay as well
because we have seen several relays and relay connectors go bad.
To sum up i think curent going thur ign because it is restricted it heats the terminals/relay and along with low grade terminals u have failures.