For developing GUI interfaces instead of 5250 green screens, SPAs seem like a solid approach.
Two of the downsides to SPAs indicated in Santiago Valdarrama's blog
(poor search engine optimization, and lack of support in older
browsers) which you referenced would not deter from replacing
5250-based DB inquiry, maintenance, and transaction processing
applications. In most cases, you wouldn't want a search engine to know
about them, and you can specify minimum browser requirements for 5250
users.
What about the "single page" aspect? Santiago Valdarrama's blog states:
"In a SPA, either all necessary code - HTML, JavaScript, and CSS - is
retrieved with a single page load, or the appropriate resources are
dynamically loaded and added to the page as necessary, usually in
response to user actions. The page does not reload at any point in the
process, nor does control transfer to another page..."
5250 applications can get rather complex as users navigate from
screen to screen. A menu item might begin by working with say credit
union "members". Users may navigate to member accounts, drill down
further to account transactions, display transaction details, etc.
Screens may change depending on whether the accounts are "shares" or
"share drafts" or "certificates of deposit" or "loans" or "revolving
credit" or "Individual Retirement Accounts", and such.
Santiago Valdarrama expresses concerns about the performance of
initial page load, if all foreseeable must be downloaded at once.
midrange.com
