Mahmood,

May I ask why you think it is a "big mistake"
after all, an off the shelf WDT has essentially the same circuitry contained internally. With the exception of the low voltage or brownout threshold circuitry. If you want low cost, the LM555 is the way to go, if you select the resistors properly and choose 1% resistors for one-shot circuit, I don't see the problem.

Jacob