Technology moves so fast that perhaps tech patents should be given shorter terms. As it stands, the big companies just build up huge patent portfolios which discourage competition.

The original patent still expires. The problem is when the patented improvement is obvious enough that anyone who wants to build on the expired patent is going to want to do it the way the new patent does it (ie. patent N+1 is "just" a modernized reimplementation of patent N), but un-obvious enough (to a "person of ordinary skill in the art", superficially at least) that it is still patentable.

Alternatively, the original patent holder throws money and people at the problem and patents every variation on their original patent they can conceive of, and every N*M combination with their other patents, even ones they have no intention of reducing to practice, maybe even ones that seem nonsensical, just in case. IBM used to be notorious for this.