Except that's entirely unrealistic as an actual business move. Coming up with $600m per year for operations, is very different for Snap versus coming up with $4.3 billion all at once to buy another business. This is especially true given Snap's (currently) anticipated revenue growth rate.
First, the people funding Snap - the people providing the capital to buy Rackspace - would not look well upon them trying to buy Rackspace. They'd think it was a huge distraction. Snap has some capital now, but they require that for operations, so they'd have to go out and raise a new $4.3 billion for the deal (and they'd suffer a painful dilution because investors would balk).
Second, you can replicate the useful (to Snap) infrastructure of Rackspace, at a fraction of the cost of buying the business as it was valued based on a relatively high multiple.
A year from now Snap should be able to hit at least $2 per user on a quarterly basis, with 200 million or more daily active users (both conservative numbers). At $400m quarterly in sales and growing, versus the ~$150m Google / Amazon costs (which might be lower in the early years of the deal?), it's a bad business move to take a 15% to 20% dilution hit to purchase the overvalued Rackspace business. That bad value ratio gets more and more lopsided if Snap keeps growing in the coming years.