An MIT/BSD/Apache project means: people can take it, extend it, incorporate it into their totally non-free product, and they may contribute back to the original project when it suits them.
An AGPL project means: people can take it, extend it, use it for whatever they want privately, but, if conveying it publicly, the covered work must have the freedoms required by the AGPL.
Your argument is basically that many companies prefer non-copyleft, and this means that non-copyleft projects will have a larger contributor pool (from their employees). Thus, non-copyleft projects will be better off than copyleft projects.
There are a number of issues with that argument.
First, just as there are people who prefer to contribute to non-copyleft projects because it means they can use it to develop nonfree software, there are people who prefer to contribute to copyleft projects because they don't want to contribute to nonfree software. Unless you have reliable data on the exact numbers of people who like or dislike copyleft, you can't make the claim that "more people will participate in a non-copyleft project". There very well could be more individual developers who prefer to contribute to copyleft projects, than there are employees that companies will pay to contribute to non-copyleft projects.
Second, whereas it's a choice to release your extensions of a non-copyleft project, it's a requirement of using copylefted projects. Thus, a copyleft project can be assured that all copyleft-friendly developers will give back to it if they extend it, whereas a non-copyleft project has no such assurance. I believe that this is a large reason why Linux has thrived and persevered over the past two decades; Linus Torvalds agrees. [0]
Third, and most importantly, free software shouldn't be about practicality. Copyleft is about protecting free software, and thus the freedom of a society dependent on software, by enabling free software development, and not contributing to the development of nonfree software. Google, Apple, Facebook, startup bubblers all hate copyleft - what a surprise! They can't use my code in nonfree software without paying me for a proprietary license; boo hoo. Free software licensing should never be about companies' convenience, winning popularity contests, or other vacuous exercises. Free software licensing should should be about protecting the freedoms of the software - for all users now and future.
Look, there are cases of copylefted projects succeeding and non-copylefted projects succeeding. If someone wants to write and release non-copylefted code, I'm still thankful for that because the code itself will stay free forever more. However, when someone releases code under a non-copyleft license, they are making our future just as comfy for nonfree software as it would be for free software, and that's certainly not ideal. I want nonfree software developers to have as hard a time as possible, because I think nonfree software is harmful. [1]
[0]: http://www.bbc.com/news/technology-18419231
[1]: http://minglis.id.au/blog/2014/04/09/free-software-free-soci... (my blog)