"why build an monolithic mirror?" "why not make it multiple times 8 meters"

Cheap, simple design that doesn't take 20 years and a NASA to develop.

8m is a massive gain in angular resolution over Hubble's 2.3m and an improvement on JWST's 6.5m. From the our linked article:

    “Diameter is a huge lever arm,” “The larger the diameter, the better the resolution and sensitivity.”

Based on the fairing specs it looks like Starship could achieve 9m. That would be nearly double the collector area of JWST.

JWST is one, very expensive, exclusive design that perhaps only NASA is capable of delivering. What if someone other than the US wants a cutting edge "Great Observatories" class space telescope? A big, simple monolithic mirror design made feasible by super heavy lifters.