> they didn't do it solely to increase their profits
But they also did pretty obviously do it in part to increase their profits.
The cartel was formed in part as a response to the volatility of the lightbulb market (there was so much competition it was difficult for any company to stay dominant or to maintain stable/predictable profits), and was followed by a large increase of sales for the cartel's members.
Short of long-term environmental costs (which nobody was seriously worrying about in the early 1900s), there's very little reason to standardize the light output, efficiency, and lifespan of a bulb, which could instead be determined by consumer choice in a free market.
If the Pheobes Cartel was simply setting labeling standards, maybe it would be debatable whether or not there was any bad intention. But it remains a good example of planned obsolescence in part because of the excuses engineers used at the time, which were pretty clearly not the entire reason for cartel's formation, but were used to dismiss criticism of reduced competition.
It's a good lesson for people to learn: planned obsolescence is usually accompanied by a couple of engineering reasons for the tradeoffs, and sometimes those tradeoffs are justifiable in a vacuum. But planned obsolescence removes the ability for the consumer to choose their own tradeoffs and to optimize for their own needs, and instead it prioritizes making specific technical tradeoffs that just coincidentally happen to benefit the company's bottom line.