That being said, it seems to me that another antagonist that got shuffled off too early hints at why it is that people who are too good at seeing through the Foobs don't last very long. Said example is John's distant relation, Fiona Brass. The following excerpts from her Liography hint at why she never endured as a character. First off, let's look at her brief stay at the Pattermanse and why she never got along with Elly:
[Elly] seemed to think that Fiona was there to be some kind of slave. She also turned out to be a neat freak, who had an insane number of picky rules and obsessed over every little bit of dirt or mess in the house. The kids had learned the same fussy ways, too. Fiona did her best to get along with everyone, but it was hard work getting the Pattersons to loosen up so that she and Beaumont could be comfortable. It didn't surprise Fiona when Elly started getting snippy. She didn't have the guts to tell Fiona straight out that she wanted her to leave, of course. Nice women never did. They just pasted on their phoney smiles and got someone else to do their dirty work.
Sure enough, Cousin John started hinting to Fiona about getting a job and moving out. She could tell he hated doing it, that it wasn't his idea, he was just acting on orders. She didn't really care much whether she stayed or went. It wasn't much fun living in a place where there was a fuss over every scratch on the furniture or cigarette butt in the sink. But it bugged her to let Elly get away with being so manipulative.
and the children's forays to the billiard hall where she works:
It amused Fiona no end whenever John and Elly's two kids, Michael and Liz, showed up at the pool hall. They'd slouch in with scowling faces, trying to look tough and street-wise but mostly looking young and nervous. She played along, giving them free soft drinks and fries before she skinned them out of their allowances at snooker. It didn't bother her to clean them. The Patterson kids had more than was good for them anyway. She was doing them a favour, showing them that life wasn't all sunshine and roses and mommy tidying up after them.
But she didn't encourage them to hang around too long. There were some shady doings in the pool hall at times, and she didn't want to be responsible for any kid getting started on drugs or anything uglier. Michael, in particular, seemed like he could be susceptible. She kept a sharp eye on him, and more than once shooed him off home when it looked like he was being set up.
You will notice right off that she regards Elly as being a manipulative creep obsessed with things that don't matter worth a damn who was busy raising her children up to be soft, weak, oblivious and gullible in order to win at family politics. Given the smug way in which she told Connie that given her non-existent sacrifices, she's entitled to treat them as vending machines just as a means of somehow balancing the scales, I'd like to say that I admire Fiona's restraint.
That being said, it's not really a surprise that Lynn had to write her out of the strip. As Ebert said about a film in which the antagonist was more sympathetic than the nominal hero, you can't have heroes and villains when the wrong side is making the best sense. Some voice in the back of her head seems to have told her that Fiona would, owing to her being more likeable than Elly, have run away with the strip so it was necessary to remove her lest Elly end up being depicted as being a fussy gold-digger crying about nothing and raising her children to be suckers and idiots.