At least back in the early 2000s when I worked there, the expectation set by Roy was that once the legal issues versus Schroder, et al [1] were settled, that Thoughtworks would someday become a public trust. It would be very surprising to learn that the company was now being sold for the profit of an individual.
There is a mention of that in the post: "Another issue encouraging a sale has been the difficulty of creating a post-Roy ownership structure for ThoughtWorks. Roy has often talked about setting up some kind of trust to own ThoughtWorks and maintain its values into the future. But setting up such a trust implies a change of ownership, and a change of ownership involves taxes."
According to the article, the issue is not that much tax avoidance as actually paying the tax. A sale would trigger a taxable event and taxes need to be paid from the cash assets. Consulting companies don't have much in cash lying around usually, so that might be an event that could threaten the companies livelihood. Now if you sell to an investor, you receive cash from which you can pay the taxes, but if you "sell" to a trust fund, you don't.
Ex-TWer and current TW shareholder here. US securities law does not allow a company to be private over a certain number of shareholders. The owner couldn't legally offer to sell the company to all 4000-some employees, even if they all lived in the US. (They don't live in the US, which makes it much more complicated).
There are other restrictions: you cannot sell shares of a privately held company unless the prospective buyer makes over 250k per year or has a net worth of $1 million.
All of the above are SEC rules. There are numerous other hurdles.
[1] http://caselaw.findlaw.com/de-court-of-chancery/1138774.html