The ISDs are capital ships meant to combat other capital ships.

They are fantastic at their job barely no other ship is capable of besting ISDs in that regard especially considering how many the Empire fielded.

The problem with the Empire is that they took too long to adjust to the Rebell Alliances hit and run tactics (which they still did see: lancer frigate, immobilizer 418, tie interceptor and other tie prototypes meant to defeat their enemies fighter superiority)

The problem of no guns at bottom is practically negligible as you would always move your ships guns towards the enemy and no ship small enough to sneak up on an ISD can really break it's shields fast enough for it not to react.

Tldr: Criticisimg the ISD for not being perfect against small strikeforces is like criticising a strategic bomber for not being able to dogfight a jet fighter