Can I ask a clarifying question? When you say "what would convince" do you mean who or what would convince potential soldiers to join by touting the virtues of the Roman army pension? Or do you mean what assurances and/or evidence were new recruits given that Rome would keep her promise of a pension after 25 years of service?

This post from an older thread, by /u/Celebreth, might be if interest to you! Although more can always be said, it discusses the land grants extensively!