My cat's asthma is worse, inhaler not working. What's his outlook?

It sounds like you have exhausted many of the options available to you already. While the medications are great particularly at reducing inflammation in the airways, they don't actually deliver any additiional oxygen to Clark's body or blood stream. For some of my cardiac patients and to a lesser extent the respiratory patients, I recommend keeping an at home oxygen set up available not that different from how a human with emphysema keeps oxygen on hand. This often helps reduce the number of attacks as well as the need for a stressful visit to the vet everytime an attack occurs or appears like it is going to occur. I am not saying it is a subsitute for a vet, but I would speak with your primary vet about potentially getting you a prescription for the tank. You will have to build the vessel yourself but can be done so out of a large tupperware and some tubing essentially with relative ease. In terms of life expectancy, it will all come down to your ability to get the attacks under control. In theory the disease can be be managed long term, but in worst cases potentially like Clarks, can ultimatley result in respiratory distress and failure. Hope this make sense.