Any time a game is updated from a previous incarnation, invariably there are going to be comparisons made. It's inherent. However, those expectations can be mapped to a continuum.
On one end, purists who just want to see the original game on their current console, with no improvements. On the other, people who are expecting a completely updated game, with all the amenities and contrivances of other new games. Most of the people cheering for the game at announcement are likely of the former category, or perhaps are wishing for updated graphics and internet-enabled multiplayer.
The truth is, in order to be considered a success in today's market, the current, mainstream audiences must be successfully engaged, and the original buyers are either not in that set, or are only a very minor subset of that target group. The FPS market has evolved so much in the intervening time, even in the limited sense of singleplayer campaigns, that the latter point on the continuum must be targeted. Nostalgia can only serve so far.
tl;dr version: Yeah, Half-Life would flop, if placed against current competition. But the current titles are only here because of Half-Life's past success.
