Cruisers are indeed pretty good; you should absolutely have some in your fleet(s) once they're researched. But they still become woefully outclassed once you can build battleships.
Alloy for alloy, battleships give you a better ROI than cruisers. The former's heavier throw-weight and higher total hit-points are generally just too much to be offset by the latter. (One battleship will nearly always defeat two cruisers of equal "tonnage", regardless of circumstances.)
Likewise, corvettes are a far better choice than destroyers. They're faster (better response time to threats, more evasive in combat), can be equipped with missiles/torpedoes, and are cheaper/quicker to replace.
The battleship/corvette fleet meta has been around practically since Stellaris was released six years ago. And while the devs have often acknowledged the problem, and occasionally given lip service about finding a solution, no one has actually done so thus far.
The fact that Stephen has now spoken openly about addressing this issue on a few different occasions gives me a sliver of hope that that may finally change. However, I'm still keeping my expectations low, and I'm definitely not expecting a fleet/combat re-balance anytime soon. Even if he does figure something out, I don't imagine any sort of fix would be added until the November update at the earliest, with next year being more likely. (Fingers crossed, though!)
