That barrier is simply this: emigration as a retired person is much more difficult than for a person in their active working years. If you are active in any of a variety of occupations, nations are much more likely to give you residence. Every nation has a list of occupations they'd like to have more bodies working in; and the strongest recommendation is to have an local employer willing to hire you. Retired people are seen as potential burdens on all social services and generally have to meet higher requirements.
That said, if you are young or in mid-life, think about it. If you are even modestly wealthy, look first at nations offering a "golden visa"[1], a residence permit given in return for your investment in local real-estate or business.
And for pete's sake look beyond Canada and New Zealand. The best prospect I found for a US emigrée was Uruguay! If you have any Spanish, give it a careful look[2,3].
[1] https://visaguide.world/golden-visa/