Yes, Public Mobile has the right to change any of the terms of service, including rates, without notice. Changes become effective thirty days after being posted at publicmobile.ca/ plans. By purchasing service after a change is posted, including by continuing to top up your account, you accept the current terms of service. You can cancel service at any time, without penalty or cancellation fees.
PM is quite good about continuing legacy plans. Many people have plans or promos which are no longer offered, many still have legacy plans (at legacy prices) from years ago.
To my knowledge, PM has only discontinued its ancient Pioneer plans - throwbacks to its pre-Telus origins. And even then, PM made a good effort to communicate to affected customers (for months), along with special offers to move towards other plans.
Nothing lasts forever. If you get a few years of awesome savings from a plan that's great. If not, you can buy something else. And if you never bought it in the first place but just watched the vendor remove it from the store shelf then you lost nothing anyhow.