Legal Question in Landlord & Tenant Law in California

Not if the landlord has a copy. A true and correct copy of a written contract can pretty much always be used if the original is lost. If there is no copy of the written agreement, that could delay an eviction, but not stop it. If there is no proof of a written agreement, then the tenancy becomes a tenancy at will at whatever rent has been paid in the past. If that rent is not paid going forward, the landlord can still evict - no written agreement necessary. Even if the rent is paid, the landlord can evict after giving 30 or 60 days notice (depending on how long the tenant has lived in the premises). So sooner or later the landlord will be successful in getting the tenant evicted regardless of losing the original written lease.