Intervals vs Span guidance

[Ramsey Haddad (BLOOMBERG/ LONDON)]

We are building our needed customizations/extensions on Solr/Lucene 7.7 or 8.0 
or later. We are unclear on whether/when to use Intervals vs Span.

We know that Intervals is still maturing (new functionality in 8.0 and probably 
on-going for a while?)

But what is the overall intention/guidance? "If you need X, then use Spans." 
"If you need Y, then use Intervals." "After the year 20xy, we expect everyone 
to be using Intervals." ??

Any opinions valued.

Thanks,
Ramsey.