Matsimus talks here about the upgrading of tanks, especially continual upgrading. He argues that the hulls eventually succumb to old age because of the stresses placed on them, regardless of whether or not they see combat service. He also makes a good point about how the armour is weakened due to holes being drilled in it or it being subject to blunt force trauma.
I think that the suggestion of upgraded M-60s has problems because of what Matsimus has said. The M-1 Abrams being sent to Ukraine isn't a good idea either because of its maintenance and fuel requirements. If this was peacetime, then yes because the Ukrainians would have time to induct it properly with maintenance, spares, etc., being taken care of. In wartime it's far to much of an issue because the Ukrainians don't have the required time. If it had a diesel powerpack then the powerplant maintenance wouldn't be such an issue.
In my view the Leo2s are the best option because of their powerpacks and the closeness to support for them. I regard the arguments about Germany not wanting to send them to Ukraine because of the suggested bad look of flying turrets would be harmful to sales opportunities as somewhat spurious and ill considered. If anything, the Leo2s performance against a near peer enemy would enhance its sales potential, if it performed well. Tanks are bought to go into battle and do unto the enemy before the enemy gets a chance to do unto them. They aren't acquired to sit on a parade ground and look good. If you acquire tanks purely because of the ascetics, then you have more money than brains.
IMHO Germany is reticent about their tanks being used because of WW2 guilt. Their history has produced a blind spot which they adhere to with stubborn determination. The sight of German built tanks fighting Russians scares them because of they don't want a repeat of images of German tanks racing across the Russian steppe. It's a psychological thing that has a hold on their political elite. Yes, in WW2 Germany did some terrible things, but todays Germany isn't that Germany.