I would agree that Web is a poorly designed spell, but surely we must be talking about more than just Web and Grease and Glitterdust?
An effective offensive spellcaster needs 2 things: line of sight and information
In very vanilla scenarios those might seem to be one and the same.
Line of sight can be denied by magical (or natural) fogs, darkness, or walls, or reduced drastically by the terrain itself.
Information can be controlled by hiding the nature of the opponents, hiding who is the real threat and who is an expendable mook, using illusions, or manipulating the pace of combat. The last one requires some subtlety. If the opponents can feint...or the opposite, the PCs may find their standard tactics do not work so well. Eventually the party will hit a level where the opposition could fog, Web, Stinking Cloud, Sleet Storm, or wall in order to retreat. And you can always resort to SR and mobility enhancing magic as a last resort (although I would consider this a bad habit for a DM to do all the time).
OTOH the PCs are supposed to fight unfair the majority of the time. FREX, surely a non-uber party could not clear out the G-series by fighting fair toe-to-toe with a small army of giants.
What is important is to make sure things do not feel like they are falling into a rut.