I agree. Provided they are good quality and haven't been overdriven or physically abused, 8 year old speakers should still be usable for many more years Speakers can be repaired since the components that wear out can often be replaced. Good quality 8 year old speakers are still very saleable and could be traded in at some audio stores.
Receivers are another issue. All receivers, including high end units, can deteriorate or fail after just 5 to 10 years. 15 years without repairs is good. Repair is an option but even very expensive receivers in working condition can be worth only a few hundred dollars after 15 years and cost as much to repair. The Yamaha RX-V1000 in working condition is worth about $100. In addition, advances in technology makes receivers become obsolete much more rapidly than speakers.
For the $500 to $2000 that a high quality soundbar would cost, there are a number of very good receivers available. Unless there are other factors driving the move to a soundbar, I would get a new receiver.