You need to have optional areas to reward exploration, but that’s not the equivalent of “the whole game is optional”. Even BotW where 99% of the game is completely optional you and everyone else still have to complete the following steps: activate the first tower, clear 4 shrines, talk to the king, kill Ganon. Minecraft for example is an “everything is optional” game and plenty of people would argue it’s less of a game and more of a pure sandbox because you have to set your own goals and do your own thing and you’re done when you don’t feel like playing anymore. You can’t “finish” Minecraft.
A game designer will always need to create some mandatory parts if they want the game to have an end state beyond “you died”. Mandatory parts don’t invalidate exploration except when “everything” is mandatory.
I don’t consider it unethical. For example if my father dies and I inherit his house where I grew up, he grew up, his father grew up and his grandfather built. That house has a lot of sentimental value in it. I have settled down very far from there. What am I supposed to do? Throw away the family legacy or uproot my entire life?
I think as long as I don’t rent it out it’s acceptable to own it. It’s just extra cost for me to keep something of sentimental value in the family. I’d even be okay with paying extra tax on it considering I think every house you own that you don’t live in should be taxed extra.