Didn't one of his calendar slams happen in the open era? I think it did
Anyway, before the open era tennis was so restricted to club goers, rich, etc., that it probably excluded the bulk of the great competition. So it makes sense to not look at their achievements the same way as when everyone who wanted to participate got in through their own steam for the most part
Yes, one of his grand slams was in the open era, but IMO that isn't enough to put him in the running for best player of the open era.
I agree that it makes it difficult because of the way it works, but if you factor in the "level of competition" it makes it difficult any way you look at it, even open era only. Add in new racquets, the popularization of hard courts...obviously the whole discussion is meaningless and subjective, but it's just for fun, so for fun, why not add in the old players?