～と言ってもいい

About

The phrase 「と言いってもいい」 can follow a verb, adjective, or noun, and expresses that "one could say X". The てもいい here is the same てもいい that is used to ask for and give permission, except that in this case, it is more general permission pertaining to people in general, rather than individuals.

When ～と言いってもいい follows a noun or な-adjective, they must have だ attached in order to connect with the と particle. The と particle marks what it attaches to as an indirect quote (i.e. the X in "one could say 'X'").

Summary

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  ～と言いってもいい means "one could say X", where X is an indirect quote marked by と