Why the Mitzvah Depends on the Eruv Being Valid
An eruv is relied upon by people who do not inspect its construction or study its psak. That makes validity the first communal responsibility.
The public acts on the permission
Once an eruv is announced, families carry every Shabbos without inspecting the route or studying the psak. They are entitled to assume that competent Rabbonim have resolved the halacha and accepted responsibility for the system.
If the enclosure was never valid or later breaks, the habit of carrying can continue long after the basis for permission has disappeared. That is why uncertainty cannot be passed from the builders to the public.
Every required part must be present
The status of the streets determines what kind of enclosure can work. The physical boundary must then be continuous. Eruv chatzeiros and sechiras reshus address the communal domains inside it, while inspection preserves the accepted construction.
Each requirement answers a different question. Strong walls cannot replace the communal joining; shared food cannot close a breach; weekly inspection cannot cure a route that was never halachically valid.
Validity is a continuing communal responsibility
A city changes constantly. Utility poles are replaced, buildings are demolished, roadways are widened, and new openings appear. A public eruv needs a Rav and inspection system able to respond before the next Shabbos.
When a route cannot be repaired, clear notice must reach the people who rely on it. The mitzvah of eruv includes guarding the public from carrying when permission is no longer in place.