I thing this can be kill Steem in a unlucky ( middrange) Bullrun.

I don't see that at all. In a bull run, inflation is actually reduced by the presence of SBD. The tougher situation is in a bear market.

This is true of DAI as well, because ETH (and other assets with MCD, but that's small so far) will be liquidated from CDPs and reintroduced into the market. That's effectively higher inflation. We just haven't see how things play out when DAI is large yet.