Yes, you are right. The more upgrades you get the less profitable it becomes (in %). However, that does not dispute the claim that the ROI will go down together with the motivation of new players. The current system is unsustainable and the rewards will slowly decrease to zero. And those who invested early win more than those who invested after them. Plus those who invested less have better ROI.
And the fact that smaller investments get better ROI can favor bots distributing a big investment into many accounts. Maybe this is already happening and many of Drugwars players are actually bots.
I am OK if people play it for fun. But majority of articles about it are just screaming it is the best investment ever.