I know Onecest (Onceler x Onceler) was a dark time in Tumblr's history, but when it serves as a metaphor, it actually makes a really good point.
Every movie can be viewed through 'critical lenses' or basically, with mindsets. For example, you can watch The Truman Show (1998) with a focus on 'what roles do women play and do they really have power' and quickly come to the conclusion that a society centered around a male (Truman) really holds no place for women (all the women in his life were easily replaced, cast out, or held no real importance besides keeping him there). With this in mind, you can look at the Lorax (2012) with a focus on the relationship between nature and greed, and money and power.
In the Lorax, we see corporate greed, driven by a thirst for power through having more money, repeatedly disrupting natural order. First it happens with the Onceler who cuts down all the truffula trees so he can become rich, then by O'Hare and his monetization of air, and it presumably happens in other situations like the Oakamatic (fake trees). I'm gonna focus on the Onceler in this though.
Throughout the movie, the Onceler serves as a metaphor for capitalism. He starts out innocent enough, like an entrepreneur, just trying to get his work out there. It's within control and no natural order is really being disrupted, just like the intent with capitalism. He's only taking some of the tree's fluff and nothing is getting consumed beyond any means. However, that all changes when greed steps in. Once the Onceler gets greedy and wants to become rich (his descent documented through the song 'How Bad Can I Be?') natural order suddenly becomes sacrificed. Now overconsumption is an issue, he's failing to replant or replace the losses, factories are being built where they shouldn't be. As far as the movie and analogy goes, the Onceler illustrates how quickly capitalism can descend from being an orderly and effective system, to a breeding ground for destruction, therefore tying back to one of the movie's themes: Corporate greed disrupts natural order.
How does this relate to Onecest? Glad you asked. If we establish the Onceler as a symbol/metonymy for capitalism, it's quickly made clear: capitalism can only love itself. This ship originated because the audience was never given anyone plausible to ship the Onceler with at any point in his story (aside from the Lorax but that was beyond cursed both as symbolism and as something to witness). Similar to capitalism, it can't love anything. It does not benefit the young or the old or nature or humans or animals. Capitalism can only realistically love itself, which is why it survives in a society like the USA today.
We can take this a step further. Since Onecest was based around the young Onceler and the slightly older Onceler, one can further this analogy by saying: Those supporting capitalism under the belief that they can make it (young Onceler), love capitalism because of a deep-rooted desire for money (older Onceler). Then ultimately, at the present time of the movie, the Onceler is old and deteriorating, not shipped with anyone, to show that any relationship with capitalism will not end well. He had the money, he had it all, he was a big flashy billionaire but he still ended up alone because his greed went too far. Like a snake eating its own tail, capitalism eventually self-destructs and leaves nothing but memories and scars behind, like the field of dead trees surrounding the old Onceler. Modern capitalism is a breeding ground for exploitation and greed, and often is irreversible. Look at the declining fish populations, the lack of Old and Mature Growth forests, the exploitation of workers in underdeveloped or developing countries, all of it was caused by greed overtaking the concern for natural order - under the ironic guise of Darwin's theory of natural selection (the stronger wins). Just like the Onceler was in his song, the modern understanding of natural selection in relation to capitalism is sorely mistaken, which may indicate that the two do not actually go together.
Ultimately, the Onceler's drive for money and power, at the expense of natural order demonstrates the effects of corporate greed, and his interesting relationship with himself due to the fandom's need to ship actually serves as an amazing metaphor for capitalism.