So let's say you want to make a living off writing the next popular language. Assume "making a living" means the equivalent of reasonable pay at a full-time job (any job—not necessarily a technical one). If you really love what you do, you'll probably accept a little bit less to do it, so let's say $12.50 an hour, or $2000/month.
While working a stable job, you release your first version and set up a facility for donations. You then engage in continuous development and marketing, to a degree proportional to the number of users of your language. Let's estimate conservatively that 1% of your users will donate, and each of them will donate an average of $1/year.
That means that in order to get your $2000/month salary, you need to have 2000 donating users for each of the 12 months in the year. That's 2.4 million users total. Let's now assume that it takes 10 years for a language to become this popular: you must therefore acquire an average of 240 000 users per year, or 20 000 users per month.
If you're working the equivalent of full time (160 hours/month), your promotion strategy and implementation quality must be sufficient to gain an average of 125 users per hour. And that's repeat users, of course: if 20% of the people who try your language become repeat users, you actually need a conversion rate of 625 people/hour.
Even if every one of the people you convince directly convinces four more people to try your language—and for the sake of simplicity, assuming that they don't go on to try to convince others—then you're still back down to the 125 users/hour figure.
Now, this may seem totally unreasonable, but believe it or not it can still work: say your marketing strategy produces roughly linear growth over the 10-year period during which your language is gaining ground, and then plateaus. That means at the beginning you'll be converting an average of 0 users/hour, and 10 years later you'll be gaining 250 users. (Again, hourly. Perspective, here.)
That's an average increase of 25 users per hour per year: at the end of each year, you're converting 25 more people per hour—or 4000 more people per month—than you were at the start of the year.
So let's revisit that 2.4 million users ballpark: is it feasible to gain that many users in 10 years? If we accept the (inherently flawed, but usable nonetheless) statistics offered by Langpop as accurate, we get the following information about the top 7 languages currently trending through Yahoo search. If one result is accepted as representative of one user (I know, bear with me), these numbers indicate the rounded approximate average number of users gained per year since the language first appeared.
- C++: 500k
- C: 400k
- Java: 700k
- PHP: 400k
- Perl: 150k
- C#: 300k
- Python: 150k
This puts things back in the realm of possibility: if you make a language that's as popular as, say, Python, then in 20 years you will have enough users to make development and support (and marketing!) of that language into your full-time job.
Make one as popular as C#, and you can do it in 10. Cool!
…Except of course that putting it that way trivialises the vastly unlikely and difficult undertaking that is making a language so popular. But hey, if you've got a good idea, and you can manage to get to the top entirely on your own, without the contributions of any other developers who would take a cut of your donation money, then you're a genius, and you deserve it.