It seems to me that this is a two part problem: helping users to know what is expected of them and also allowing for commitment tokens to be freed up in some way other than the fulfillment of their commitment to the site.
In an answer to a question here on Meta the following is mentioned:
To fulfill your commitment, you have to post a combined total of 10
questions and answers and participate for three months...
These requirements are quite clear if you can find them but for the most part they can be really hard to find and the de facto policy seems to be to not inform users as to what is expected in hopes that it will encourage them to contribute to the betterment of the site. Personally, I think this might be the wrong way to address the situation as people generally want clear indications as to what is expected of them.
As such, my suggestion would be to make the question and answers requirement clearer, but also extend the requirement to require that they be "good" questions and answers that are up-voted by other users of the site. This would serve two purposes: users know what is expected of them and since they must be up-voted it ensures they put some thought into what they are doing and it also encourages users to up-vote questions and answers on the site. Both of these would be a good thing for a new site.
Next we have the issue of commitment tokens being locked in which can cause people that may not be able to fulfill their commitments from trying to work on another site where they might be able to make a better, positive impact. Obviously some work would be needed to ensure that people don't just game the system, but you also don't want to penalize people that recognize their own limitations and what to bow out accordingly. Case and point would be my experience with the Japanese.SE site, I tried to do what I could in the beginning but as the site moved in the direction of requiring knowledge of hiragana and kanji that is beyond my current capability I'm unable to contribute to the site in a positive fashion.
As such, my suggestion for handling this would be in line with what I suggestion in an answer to another question: penalize users that make no effort but allow others to bow out with either a nominal or no penalty hit after a certain time criteria is hit (i.e. 90 or 180 days in beta). As others have noted, sites can stay in beta for an extremely long time, the aforementioned Japanese.SE site has been in beta for over 225 days and there is no clear indication that it will be closed or moved forward any time soon. Thus, by locking commitments for a specific amount of time we encourage users to contribute to the site in a positive manner to release their tokens early, but also allow for them to move on if they find themselves unable to contribute for a variety of reasons.