When the number of idle sites are order of magnitudes larger than the number of active sites at any given moment that solution is equivalent to running PHP as CGI. That would price you out of the market. (And spare me the morals please, as much as I'd like to see mod_php dead it is still an important market which drove PHP's success.)
PHP won because of mod_php, but mod_php won the low end market because of safe_mode. My question still stands, what should replace it? Maybe SELinux can?