I'll say!

Not modern slang, but I certainly use it, so it's not dead yet...quite.

Thank you, Mister Micawber.
Would it be okay to say “That's right. Nearly every day” instead?

'That's right' does not carry the same level of emphasis; it is merely a confirmation.

HUBLOTIs "I'll say" really an old-fashioned phrase?

It sounds old-fashioned to me, but not so old-fashioned that I would be surprised to hear someone say it.

CJ

Thank you, Mister Micawber and CJ.

What would you say instead of "I'll say" in “I'll say! Nearly every day”?

HUBLOTWhat would you say instead of "I'll say" in “I'll say! Nearly every day”?

I'll have to think about this for a while. I don't think I use any kind of equivalent of "I'll say" in that situation.

CJ

Does she see him often?
You bet! Nearly every day!

HUBLOTThank you, Mister Micawber and CJ.What would you say instead of "I'll say" in “I'll say! Nearly every day”?

Some might use a jocular question such as:

"Is the Pope Catholic?"
"Do bears sh*t in the forest?"

The answers to such questions are so emphatically 'yes' that the point is made.