it seems weird to spend $10billion on a telescope that lasts for 4-5 years and then not spend the money on a communication system that isn't limited by traffic and is 'expensive', or a spare drive to double its lifespan.
It's kind of important to remember that they didn't intend to spend $10B on the telescope and that it's going to last for ~20 years. The original price was expected to be $500M and it would have never been approved if the original price was $10B. It only ended up being able to get to such a cost because of gradual overruns over 20+ years and sunk cost.
That said, NASA have been expanding the DSN for the past decade with four new antennae added so far (remaining two expected by 2025), it's just slow progress as usual with most of what they do lately. DSN antennas don't bring in jobs (and thus votes) for senators the way decades long flagship projects like JWST or SLS do.
Lifespan is not dictated by hard disk space, it's dictated by consumables like liquid helium, other cryosystems and fuel for stationkeeping thrusters. Most big NASA projects are specced to just promise enough to get funded (in terms of science) and overengineered, so what gets built will often be stretched longer than the intial deliverables demanded anyway (see basically all the recent Mars rovers).
I don't think you understand how much "billions of dollars" is for construction projects. We're talking a communication system, not the large hadron collider.
