Although seeing as how there's no party in the US that would cater to social conservatives and fiscal liberals (and vice versa), your statement is mostly correct.
The drama of something to argue over with their Establishment sister party, perhaps?
It is plain that the Democratic Party doesn't want Sanders voters if they're going to demand Sanders policy outcomes. There's no drama in successfully helping people, and what kind of partisanship isn't made of drama?
It is plain that the GOP didn't want Trump voters until Trump more or less took over the ticket with the help of professional-class journalists looking for... drama.
I think that it hurts the GOP more than it directly helps Democrats. In my mind a sudden large increase in voter turnout may give sway to candidates running as Democrats that may not be totally in line with the DNC as a whole. You're seeing a huge leftward push by Democratic voters in the presidential election, and while I don't believe it's as fragmented as the GOP currently is, I don't see any reason why this split won't continue.
Probably the most under-represented group is the youth vote, so 100% turnout would probably mean much more progressive, socialist results as well. I say socialist here without the negative connotation.
> If that were true, why wouldn't the government try to push mandatory voting laws when it had leadership from the Democrat party?
Because the Democratic Party prefers to expand voting by expanding access (making it easier to register, and easier to vote by expanding times, methods, etc.) rather than by personal mandates and threats of punishment.
Although seeing as how there's no party in the US that would cater to social conservatives and fiscal liberals (and vice versa), your statement is mostly correct.