The increased resolution is extremely important given that the diffraction threshold is function of the wavelength and the mirror diameter. And you can clearly

The increased resolution is extremely important given that the diffraction threshold is function of the wavelength and the mirror diameter. And you can clearly see that in the MIRI images at a longer wavelength that have a noticeably lower resolution compared to Nircam. If Webb mirror was as big as Hubble the resolution would have been bad in the long wavelengths. Hubble couldn’t have had a better resolution with better instruments, he was already limited by its aberration problem and the new instruments were designed to mitigate that problem.