Alternatives to the WWW are books, lectures, videos, movies, notes, compact disks (really the same thing as the WWW if hypertext is used), audio cassettes, and demonstrations. Each of these may be as good or better than the WWW in some instances, but all suffer from either predetermined pacing or poor interaction, especially when used by more than one student. The WWW is highly interactive and has perfect pacing for each individual.
Authoring for the WWW does not yield financial rewards now but could if certain pages are closed to those who do not pay for a password. Most teachers, even those who write textbooks, earn little from their writing anyway.
The WWW is shared. No one person has to write everything. Much of what is needed for teaching may already be on the WWW, and the amount of available information is exploding. More time may be spent in the future sifting through redundant pages to find the best.
The WWW provides inexpensive and highly effective teaching. Much can still be learned about how best to use it for education, but it is pretty good now. Authoring is fun.