Transformers have no moving parts in direct contact with other solids, do not rely on non-reversible chemical reactions, and do not rely on nuclear reactions. So, with inert materials, the atomic bonds that make up the transformers should theoretically never change after it is manufactured.

As far as I know, commercially produced power transformers age because their insulation between the sets of windings is made of kraft paper (primarily cellulose) and their coolant is made of mineral oil. Both consist of complex organic molecules, which not only are reducing agents strong enough to be used as a fuel in the presence of an oxidizer, but also chemically self-decompose over time. That is further made worse by water ingress into the coolant as the seals (made of rubber) degrade over time. Also, unlike in thermionic/incandescent filaments or integrated circuits, the current density in a transformer winding is low enough that electromigration should be too small to even be measured by electron microscopes.

So, under current design and manufacturing methods, how long can a mass-produced electrical power transformer be realistically made to last at rated load without maintenance? Under the given conditions of 1. that transformer is made entirely of inorganic chemicals that are inert with each other, 2. there are no deaign or manufactueing defects, 3. the coolant never comes into contact with an oxidizing agent, 4. the transformer is not damaged or destroyed, and 5. the power supply is always maintained, will the transformer last at full-load rating until the Heat Death of the Universe?