This is an excellent point. Turnout may be an important signal which is lost with compulsory voting, and certainly any politician trying to suppress votes is dishonest.

To play devil's advocate and to argue for compulsory voting for a moment, however: It is a potential antidote to the type of voter apathy by which a somewhat demoralized but popular party (e.g. Democrats in the last US election) put themselves out of power by not being as motivated as a smaller group.

So while knowing the width of the motivation gap is useful, a polity could also just compel everyone to vote and then ask if they would have done so if not compelled, to get that information. Which is worse: Inaccurate data about voters' excitement, or sudden and surprising political shifts because people were sure they didn't need to go out to vote because they assumed their party had the majority?

Without compulsory voting, only those with strong motivation will vote - and there is no guarantee that an outsized motivation on one extreme side of the spectrum will not drastically and surprisingly take over if most people felt safe staying home. Austria right now seems to be a good example.