Without replaygain your loud songs have a small dynamic range and your quiet songs have a large dynamic range most of which is below the noise floor. With replaygain your loud songs have the same dynamic range, your quiet songs have the same dynamic range, but a larger proportion of the latter is above the noise floor, meaning you've effectively increased the dynamic range.
Only if they're clipped after raising the level, which is often unavoidable but also often not noticeable for the brief transients that end up getting clipped.
In the end, digital volume leveling can't be lossless. So you don't have to use it. But for mixed playlists, it's very nice.
The argument of whether it would be noticeable is a valid one, but it also weakens the strict dynamic range argument. All you can do is use a limiter, which is a special case of a compressor, which is so named precisely because it limits the dynamic range of a signal.
