There will always be sales of the PHB1, however small in number they might be, so long as it is still made available and 4E is still the current edition. Yes, you can basically not need to buy it if you sign up for DDI, but not everybody wants to only rely on DDI. Some of us actually like having physical books.
Wotc will do reprints with erata incorporated if they figure out that there is enough demand. It seems though is that there are more PHB1s than the market wants allready. I find it very difficult that Essentials will rise the demand of PHB1s to the level of having Wotc to do any re printing. Wotc will simply market new books that both new and old customers (customers that have allready bought PHB1 for example) will buy.