I'm 66 & got breast implants in '04. There is hardness/encapsulation-no other complications. What problems might occur if I never have them replaced?

Can get worse. The capsular contracture can get worse and eventually cause a significant amount of discomfort. Talk to your surgeon about either removing the implants and/or performing a capsulectomy. Although age is not a contraindication for surgery, it is better to have it done at 66 rather than at 86.
Contracture. If you are developing capsular contracture, it may continue to evolve and breasts may continue to feel harder with time. If it gets to a point where it becomes uncomfortable, then you will need to consider treating it.
Your ok. Capsular contracture can occur with implants. Many people live with some degree of it. In and of itself, the contracture is not harmful. However, if the contracture worsens, the implants may become asymmetric, palpable or even painful. As a result, they may need to come out.
Stable or progressiv. It really depends on whether your contracture is stable or progressive and if your implants are silicone or saline. If stable there is very little to be concerned about. If progressive and the saline there is a greater likelihood of rupture and deflation. If silicone, the material could develop into an intra capsular or extra capsular rupture. This is based upon potential for crease fold failure.