If you say you will go to visit a castle, then you should be excepted to see a historical building. But in Japan, that may be the case. Japanese like to rebuild the historical buildings, maybe because there is a lot of earthquakes. Most of them will restore to use traditional construction methods.
However, Osaka Castle is a "modernize" Castle. It rebuilt by ferroconcrete in 1930. Moreover, in the 1990s, even it got more modernizations. It upgraded to be a modern museum, including installing elevators.
For us, with a young family member, that is very easy to visit. My daughter can sit in her pram during the visit