Figma was a huge threat to Adobe's dominance

Instead, they choose to be slumlords.

Adobe, Oracle are just the best examples

Lets say I have a company and want to develop some capacity - I could start from scratch, or I could buy a company that already does it well. If everyone starts from scratch that would be a whole lot of society's resources spent doing repeat work, but it also represents an opportunity to learn new things and try new strategies.

hard to say if its a net good or bad. but restricting companies purchasing companies probably wont happen.

The first sentence, I agree with the general sentiment, but it's just not realistic. It would have to apply to everything; from small business owners all the way up to corporations. Buying and selling companies is why free-market capitalism has continued to deliver results. Nothing needs to be changed about that.

The problem we have is a government antitrust regulation problem. We would not be here if antitrust law were implemented diligently these past few decades. Adobe would be incentivized to compete fair and square.

For example, how do you stop a company from buying publicly traded shares, which is basically buying a company.

And if you do stop companies from buying publicly traded shares, the entire stock market collapses. Individual buyers buying shares from their personal accounts form a vanishingly small percentage of stock trades. I suspect even a lot of the individual purchases are done through corporate entities for tax, liability, and bankruptcy protection reasons.