Great question, the answer is maybe. There are a number of variables at play. The first thing to note is that SP is not a liquid asset, and can only be withdrawn over a period of time. The witnesses are rewarded with "vests," which are essentially shares in the platform and are represented by SP. So, in actuality, while the SP they are receiving is an asset, the rewards they receive for producing blocks is more akin to a stock option than a salary.

Second, the value of STEEM is currently $6+, but the cryptocurrency market has fluctuated wildly within the last year, with STEEM value dropping as low as $.25. If the market drops tomorrow, $40,000 could be $2,000 and with a lack of liquidity, the value of each witness's holdings would be negatively impacted.

Third, to answer your first question, the number of vests per block has changed several times over the course of the platform and could very well change again in a future hardfork.

Finally, when it comes to the top 20's, their costs are much higher than that of an average witness. They have usually been selected to their position because they are running multiple services for the platform, which include databases, APIs, public nodes, semi-public nodes, third-party services, etc. and the costs to run these services on multiple VPS's with redundancy for network and system failures is enormous. This cost continues to grow exponentially as the blockchain grows and the demands on these servers become higher.

Hopefully, this answers your questions, and thank you for reading our work!