Obviously not. What would happen in that case? The Google Shopping unit would go to the CFO (or whoever is responsible) and tell them that they needed more funding for bidding on carousel spots, so as to increase their market share. They would make the very solid business case that any money spent there would end up back in the company's accounts, so it is actually free. So, they would get whatever money they would need in order to outbid everyone else.
As long as your expenses flow back to the entity that's funding you, there won't be any limit on funding.
The product results that appear in Google Search are called Product Listing Ads (PLAs), and are not a comparison shopping feature today. (I don't know how things worked in the past.) They are simply advertisements.
Individual merchants publish their product catalog into Google's Merchant Center, along with the price they're willing to pay for each product ad. It's similar to adwords, just with product information too (title, picture, price).
Consequently, it's not a comparison shopping system - within Google Search it is purely an advertising feature with more information than text ads. Furthermore, as far as I'm aware, any company can participate in it by listing their products. I also think but am not positive that the only products featured in the carousel will be paid advertisements - another way in which it is not a comparison shopping system.
Note: I'm only describing Product Listing Ads within Google Search results, not whatever else Google Shopping might be. It does appear that the main Google Shopping website also involves payment from merchants, which they disclose on the page.
I don't know what determines whether the product carousel is displayed or not. I would assume that it's shown at the top when Google has enough information to display it, and when it's outperforming other types of ads. Anecdotally, Product Listing Ads have considerably higher conversion than traditional AdWords, so I would expect that PLAs will naturally perform better and would justify the top advertising slot.
The product carousel shows only paid ads, so it doesn't really matter that it's part of Google shopping. Would it be ok if the actual "Google shopping" didn't exist, and it were officially just a carousel of ads?
I'm also quite certain that they internally charge a business unit such as Google Shopping for the placement. Would it be ok if they allowed other comparison shopping sites to bid for the carousel spot?
It's complicated by the fact that for most products, Google does actually provide a much better experience than competitors. Take flight search, where the Google interface is just so much better than any competitor. I get that it can be in the consumers' long-term interest to preserve competition even if it means hobbling the best product, but the magnitudes involved here currently make it a really difficult decision.