I agree with what you're saying that exponential growth eventually stops once everyone has been infected. The unknown number is how many people have been infected so far, to quote dan:
Low estimates suggest 6% of people have had it in UK overall but 15% in test capital Germany - I'm hoping 20-30% in London. I pray it's higher still.
So even at 30% there are still an awful lot more people 'available' to catch the virus & you'd expect to see exponential growth of death figures to continue. That the latest figures suggest it's slowing... well lockdown *will* have reduced the infection rate, so in the absence of something else being shown to reduce it (or to reduce the death rate amongst the infected) then lockdown seems the most likely explanation.