You are assuming the stock market is rational (whatever that word means.)You're assuming Softbank can get anything like the $32 billion they paid. That's a pretty big price for a company with REVENUE of only $1.6 billion, and that already dominates licensable CPU cores to such an extent that future growth can't equal past growth (and free alternatives like RISC-V seriously limit their ability to collect much money in future low cost markets like IoT)
You can "craft" many stories simultaneously of how ARM is a growth business but also it will face more competition so it may be larger than today but lose market share. The future is uncertain many of the stories we tell can be true.
And depending on which stories the people who would buy ARM believes will determine the selling price of ARM. Or you can go IPO where ARM will be priced at $40 billion or higher, yet it may go higher than that or lower than that when the IPO actually happens.