Personally, I sort of like the idea of the proposed change, but just to play Devil's Advocate, let me defend the Class A/Class B/Class C status quo ...
Class C Starport:
Can manufacture, service and repair small craft. These capabilities also enable it to perform many repairs on Adventure Class Ships (which for this discussion, let's define as Starships in the 100 to 1000 dTon Range capable of landing on a planet's surface.) This paints, in broad strokes, an image of a starport designed to serve a world capable of reaching space, but with no real need or desire (or lacking the resources) to exploit the star-system as a whole. A typical Class C starport would have only a downport (no highport). It would have access to orbit via small craft to accommodate the arrival of any ship incapable of landing on the planet. The Planet itself would probably be focused on cis-lunar space, operating almost exclusively within the 100 diameter sphere of the main-world. Thus it might have orbital farms or solar power satellites or even lunar mining. However, the dominant focus is on the main-world, and especially the surface of the main-world.
Class B Starport:
Can manufacture and repair small craft and boats (non-jump capable ships). The starport typically includes both a downport and a highport. This is not by accident, because the Starport is also a major spaceport on the main-world of a system where the entire star-system is economically linked into a vast commercial web.
Gas giants are home to massive fixed fuel processing and petrochemical commercial outposts that are part space settlement, part highport, and part industrial complex ... large 'company cities' floating above the Gas Giant. Small Craft shuttle people from the main-world to the gas giant in regularly scheduled commercial flights. Giant tankers shuttle unrefined fuel and petrochemicals in bulk to the waiting markets on the homeworld.
The Asteroid Belts bristle with prospectors looking for the next big strike, and Industrial Conglomerates with operations that steadily chew asteroids and spit out common and special alloys. Carbon, a waste in most places, is sold in bulk at the gas giant to manufacture hydrocarbons using the abundant local hydrogen. In turn, the mining operation has a vast craving for the unrefined fuel to keep the power plants running.
Transporting these supplies back and forth requires a lot of boats and small craft, that need maintenance and repair and eventually wear out and need replacement. That is what supports a Class B starport in this system.
Class A Starport:
The best of the best. The shipyard can manufacture any starship you might need, and with good reason. Like an Airport that is a Transportation Hub, this Starport includes a downport and a highport that are not sized for the local planetary needs, or even the system needs, but for the larger regional needs of the subsector. The world and starport are important as a source of goods, and a destination for goods, but they are also important as a distribution node for goods. A place for vast quantities of Computer Parts to arrive and the be broken down into a dozen smaller shipments bound for worlds that could never have required the full cargo load. It is to the Class A starport that the Subsidized Merchant goes to sell what the backwater worlds produce and buy what they need. The Class A starport is the Mercantile Exchange for everything within 6 to 12 parsecs. It is an economic powerhouse beyond what the world produces or consumes.
Each class of Starport serves a different purpose and has different capabilities and has a different intangible quality beyond the simple features or game mechanics ... a starport "genius loci" (spirit of the place) ... that makes it unique.
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