Witnesses (are the ONLY ones who get "external" STEEM for their services).

Other Users (Posters and Commentators alike) will 'only' get STEEM from whatever STEEM is circulating from before the change happens - and from whatever the Witnesses decide to 'tip' into the system (in other words the Witnesses will, for all intents and purposes, become the faucet from which STEEM 'might' come forth (it is "their money" after all).

Exactly, you've got me right.

... the STEEM will quickly dry up - it'll become an exclusive club of holders - with STEEM initially going up in value by virtue of its scarcity - with Witnesses towering over everybody else by virtue of monopolizing direct access to the STEEM - with everybody else languishing (let alone new users).

For the content providers, it doesn't matter how much Steem they receive. What matters is the fiat equivalent of the Steem they receive.

Suppose that Steem becomes scarce and valuable and it appreciates from $4 USD to $40 USD. There's a writer who used to average 10 Steems per post when it was $4 USD. When Steem becomes $40 USD, that writer might receive 1 Steem instead of 10, but in fiat terms their average might be the same.

That writer wouldn't mind going from 10 Steems 1 Steem, because they don't live in the Steemland. They live in a country where they have to pay their expenses in their own fiat currency.

Moreover, I believe the tipped amount would slightly increase as the people start to tip lower and lower amounts of Steem, such as 0.001 Steem. Such amounts would be easier to be tipped due to the psychological effect.

I don't believe we will end up with an exclusive class of holders. Think about Bitcoin. It goes up and it goes down. There will always be people who need to sell their holdings, for example in case of retirement.

Moreover, think about Steems as shares of Steemit. Shareholders of companies such as Apple issue stock options to motivate their employees. This will be exactly the same system. Steem holders are incentivized to tip for good content to increase the value of the platform, which will in turn benefit them.

New users can buy Steem from the free market, like I did, or they can earn Steem through posting and commenting. Steem is not a necessary component to use and experience the platform.

... "good" content would be nothing more than those posts delivered by the highest self-tipping (highest STEEM-owning) users... which is 'not' a good indication of the true quality of such works.

I see where you are coming from, but I see this differently. Self-tipping has no financial advantage whatsoever except for exposure. On the other hand, Steem holders, i.e. share holders of Steemit have all the incentive to tip great content to the trending and hot tabs to make Steemit an attractive platform for other users.