ICAO PBN Manual Doc 9613 states:

1.2.5.3.2 Because specific performance requirements are defined for each navigation specification, an aircraft approved for an RNP specification is not automatically approved for all RNAV specifications. Similarly, an aircraft approved for an RNP or RNAV specification having a stringent accuracy requirement (e.g. RNP 0.3 specification) is not automatically approved for a navigation specification having a less stringent accuracy requirement (e.g. RNP 4).

1.2.5.3.3 It may seem logical, for example, that an aircraft approved for Basic-RNP 1 be automatically approved for RNP 4; however, this is not the case. Aircraft approved to the more stringent accuracy requirements may not necessarily meet some of the functional requirements of the navigation specification having a less stringent accuracy requirement.

Because the aircraft might NOT meet all the functional requirements → CORRECT

Because monitoring and indication systems may not be suitable for that type of specification → INCORRECT. It is not only the monitoring and indication systems, but ALL the functions such as navigation accuracy and technical abilities.

Because operation is only permitted for one type of specification → INCORRECT. One of the many advantages of PBN is the ability of navigation equipment to be able to meet many specifications - as long as they are individually approved.

Because there is only a limited number of aircraft permitted for certain navigation specification at a time → INCORRECT. Another advantage of PBN is that every aircraft could use the same navigation specification at the same time without affecting the airspace capacity.