Have you taken it all apart and cleaned and serviced the assembly?
It's hard to think the spring has lost tension, it ought to be well within its fatigue limit and capable of endless operation.
I'm not sure that this is the root of the problem either.
I assume when dérailleurs no longer drop into top, it's due to overall wear in the pivots (if all stickiness has been removed and trying it without the cable installed)
It is ultimately the weak point of the 9 speed era.
The (non replaceable) spring in the parallelogram can end up not strong enough to overcome Friction and wear as it reduces in length towards top gear.
(The rapid rise models are cheap and provide quite a few spares, as well as being in better average condition because most riders don't like them)
Shimano techdocs might show you other derailleur parts that will do the job.
If you want to strengthen the b- spring:
Some springs can be persuaded to provide more tension with a little bending, occasionally heat, but I don't think this shape lends itself.
The spring could be shortened a little and a new hook formed on the end if you have a good heat source.
An alternative fix can be to put a 1mm washer between the derailleur fixing bolt and hanger. This increases the derailleur reach - assuming you have some spare for 1st gear.
