Fanaelialae
Legend
I think that's an oversimplification. A single farmer with a sending stone is a potential target that the tower watch isn't.Sending Stones are an Uncommon item - so they cost a couple hundred gold and a couple of weeks to make (by Xanathar's crafting rules). Or a hundred gold and a few days for an upper-level artificer. But, they are permanent. Watchtowers and people to staff them cost money too - using Lifestyle expenses as a guide, we are talking about one gold a day per watchperson.
If giving the farmer a sending stone can replace the tower watch, it pays for itself in under a year.
Even assuming that a sending stone is worthless without the pair (and assuming there's no one who could acquire the other one) it's still a worthwhile target. Bandits who acquire it can potentially relay a significant amount of misinformation (no bandits sighted in the area, all clear to send the tax shipment through) before their ruse is discovered.
To be clear, I'm not saying that it can't work or shouldn't be done. It has the potential for interesting implications (like the above scenario). Just that I don't think it's as straightforward as the sending stones paying for themselves in a year. It's technically true, but only if those valuable items aren't lost, stolen, or otherwise absconded with.