Atari didn't put in the effort, but Chris Sawyer did. Now Atari paid Sawyer for the rights to the game. I do not think Atari is a parasite here just because they paid for the game instead of creating it.
It seems to me that the logical outcome of your interpretation is that Sawyer's leniency towards the OpenTTD devs would be punished by losing exclusivity to his IP. Essentially, you are asserting "squatter's rights" to IP - if IP rights are not enforced, then they lapse. This is an interesting idea in principle, but I'm concerned that it might have prevented OpenTTD from ever being created. Original creators would be incentivized to chase off derivative works to protect their IP.
It seems to me that the logical outcome of your interpretation is that Sawyer's leniency towards the OpenTTD devs would be punished by losing exclusivity to his IP. Essentially, you are asserting "squatter's rights" to IP - if IP rights are not enforced, then they lapse. This is an interesting idea in principle, but I'm concerned that it might have prevented OpenTTD from ever being created. Original creators would be incentivized to chase off derivative works to protect their IP.