What is that point in business where it is extremely difficult to generate new business/customers called?

Saturation works.

Saturated market might work even better. (https://en.wikipedia.org/wiki/Market_saturation) Means that there is no new business to be made in that particular market, a real zero profit situation if the business doesn't branch out to something else as well.

Market Pollution, or pollution simply can work too, although pollution is a passing thing.

(There probably are more alternatives.)

It looks as though you are asking for different things in different parts of the question.

1. Opposite of peaking. To peak means to reach to reach a maximum (Merriam-Webster). Then the opposite is to hit a minimum. You can use bottom out (M-W). But this suggest that sales may begin to recover in the near future, as in a cyclical business.

2. Example sentences. These state that sales are falling, not necessarily that they are rock bottom. So you could say business is declining or slumping (M-W). An example similar to yours:

Unfortunately, in the same two year period the Hays Asia Pacific business has slumped, with operating income down by nearly one third and operating profit nearly halved

3. Title. Here one can get the impression that the business simply stopped expanding, but that sales are holding on. We can then say that business matured or reached maturity (see this article or this other one).

Stagnate: (verb) to cease developing, progressing, moving, or advancing. Source

cease developing; become inactive or dull.

"Teaching can easily stagnate into a set of routines." Google search result

How about plummet?

: to fall suddenly straight down especially from a very high place

: to fall or drop suddenly in amount, value, etc.

M-W

For a candidate for a one-word antonym for "peak/peaked" in the business world there’s trough/troughed

trough verb (LOW LEVEL): to reach a low level, price, etc. before going up again: “The economy troughed six months ago and is now growing again.”

(from Cambridge Dictionaries Online)

“… sales troughed in June 2009 and have been increasing since then.”
(example usage of “sales troughed” from ‘Business Statistics of the United States 2010: Patterns of Economic Change’ edited by Cornelia J. Strawser, via ‘Google Books’)

Just as it takes hindsight to confirm whether business has “peaked” (and is now going down), one can’t know for sure if business has “troughed” until a recovery is started and maintained; so it remains to be seen if the business you describe will remain stuck in its trough (seems very likely, alas), in which case (and regardless) Rathony’s “hit/touch bottom” would get my vote for the term you seek.