Let's assume that 86 wins is reasonable for the roster we finished with. You are talking about a roster that let Randy Wolf, Ty Wigginton, and Mark Loretta go only to replace them with inferior players.
The disconnect may be contained above.

If the numbers indicate that the Astros overachieved by about 5 to 10 games last season, the 3 players listed above were three of the chief overachievers.

And although I didn't like the way Wiggy was let go and love his gritty style of play, it still stands to reason that the chief overachievers of last season are now gone. Doesn't it then follow that if neither of those players overachieve again this season, as their historical trends might indicate (or in Loretta's case his declining talent), then it's actually better that they're playing someplace else?

The question for this season is did the non-contributors from last season, i.e. Bourn, Towles, Matsui, etc, underachieve, or are they just not very good? And will Pence improve or has he already reached his potential?

Those questions alone make Clack's scenario of a .500 season at least plausible. And even if that does turn out to be optimistic, one can at least enter the season with some sense of hope of at least being entertained, even amidst all the doomsday forecasts.