But they were just paying that, in the form of paying you. If they don't think the task is worth paying that much to get done, they would have just been not-doing-the-task.
Also, note that I never suggested mentioning that the SaaS service you create should make any mention of its service being automated.
Consider an entry in the "cloud bookkeeping SaaS" genre: they sell you on having real accountants looking at your books, with automation to ensure the reliability of those accountants' work. They never mention that they'll also be using automation to make your workload easier to handle, such that each of their accountants is handling the books of 1000 clients. That's an implementation detail.
They were paying you that, sure. But then you showed them that they didn't need to pay you that anymore, because you automated that job away. So now they won't pay you nearly so much.
You're welcome. Now, get back to work, you slave! ;)
Ah, but you didn't automate the job away. Like I said, you did the automation on your own time, with your own resources. It wasn't work-for-hire; they don't own the rights to it. And there's no instance of it already set up for them to use. As of the moment that you offer them the opportunity to subscribe to your SaaS service (and note that you don't have to mention that it's your SaaS service), they're still in the position of having no automated process, just you sitting there able to do the process manually.
So they have two decisions to make: whether to buy the SaaS service (in order to receive the benefit of the automation); and whether to keep paying you.
They can choose to say "no" to both... but now, not only do they not have an automated process, and just as little idea of how to automate it as they did when they started; they also don't have you there to do the process for them, and they lose your hard-won expertise on the process that would be crucial for building any sort of in-house automation.
(I'm assuming here, note, that the company isn't one that tends to automate processes—if they did, they'd probably have already found the low-hanging automation fruit that your job consisted of. As such, they likely don't have any in-house automation expertise, nor do any in-house expertise for evaluating the competence of automation consultancies.)
ETA: my original point was that everything I'm saying here (and more) is what some person with sufficient motivation to think through all the details would be saying to their boss to convince them. "Convincing your boss" includes "thinking through all your boss's objections and having rehearsed, polished counters for them." Have I done that? No. But I'm not someone with an automatable job.
Once you've shown them that the job can be done easier/faster/cheaper, they will insist on that as the new standard.
Rinse and repeat until you are penniless and they have all the money.