If you can't fix the slowing growth and someone else wants to buy it and attempt to do so I'd say why not sell: the multiple you'll probably get on your revenue should be a tidy sum.
Looking through some listings on a broker like FEInternational (https://feinternational.com/buy-a-website/), most SAAS businesses are valued at 3-4x anual profit.
He said profit margins were 60%, so $400000 x 0,6 x 4 = $960000.
And thats for a healthy business. This one is not growing, so you get probably less then that. After the seller's fee, and taxes you might end up with something like $600k.
Even if I did net the $600k (3x my current annual salary), I wonder if I could use it to recreate something I have right now (a biz that produces $200k/year for me w/ little effort)