If you take as a basis that the Chilean right-wing alliance won the presidency (54%) and nearly 40% of the popular vote in parliament at the last general election, 20% in the constitutional convention (together with the context that the Chilean right has lost a significant number of local elections in the last year or two) is consistent with claiming it has taken a beating.
You may be right that it can form alliances with independents to block certain measures - potentially with great results if it marshals them effectively. But I don't know a party on the planet that wants to forge policy by pleading with minor parties and independents rather than deciding by itself.
You're right, no party wants to give up unilateral powers, but being the biggest fish in in a pond of all small fish is still a huge advantage. The center-right coalition list nearly half their seats, but the center-left lost actually 50% of their seats. Let's look at the numbers.
Losers
- The center right coalition lost 35/72 seats
- The center left coalition lost 26/52
- The Green Ecologist Party list their only seat
- The Humanist Party lost all 5 of their seats
Summary: right-wing groups lost 35 seats, left-wing groups lost 32 seats.
Winners
- 17 seats were reserved for indigenous representation, I don't know if there's any set political alignment to that
- 46 seats went to candidates on independents
- 4 seats were gained by the left-wing coalition, which includes the Communist Party of Chile
End result: 35 fewer explicitly right-wing seats, 28 fewer explicitly left-wing seats, 63 seats added without set loyalties. Put all together, there's not exactly evidence here that political opinions in Chile moved in any direction at all, just a lot more people voting outside of major parties.
How about the communists?
- The Communist Party of Chile actually lost 1 of their 8 seats
- The Revolutionary Workers Party went from 0 seats to 0 seats
- The Chilean Communist Party went from 0 seats to 0 seats
- The Revolutionary Left Movement went from 0 seats to 0 seats
Doesn't look like a big win to me.