I believe that you are vastly underestimating Ethereum. The value of Ethereum is not 4% of Bitcoin's. It is currently 32%, according to
https://bitinfocharts.com.
Market capitalization (aka supply*price) does not correctly represent value specially for altcoins with no cap and a massive premine.
In terms of there being no cap on the total amount of Ether, I think the point is that while Ether is deflationary, the owners are also able to stake their Ether in the consensus protocol, which will yield them returns. The average returns gained from each ETH at any given time will be equal to the average reduction of value of each ETH at any given time due to the deflation. And since the average returns equals the average reductions, the value of the asset remains constant in time (when disregarding other factors, of course) in the eyes of the investors.
A miner is running a business and has made a significant investment. Why would a businessman kill his own business by attacking the very thing that is making them profit?
Yes, this is already taken into account in the estimated price of the attack.
I'm not talking about the costs of the attack, I'm questionings its very viability.
Well, it seems that we agree that in order to bribe any existing miners, the Ethereum investors would have to also compensate them for their loss of business. Therefore, whichever strategy these investors choose, i.e. bribing existing miners or constructing new mining farms, they have to ultimately pay the capital expenditures (CapEx) regardless. And that price is estimated to be between $6B and $20B.