Twitter hasn't provided any further information at this stage as to any possible successor.
It is worth noting that it added a new 'Conversation Insights' feature to Media Studio in November last year - so could be looking to point people to this platform for future analytics.
Users now don’t need a separate IGTV app to watch longer videos. The experience is embedded in the main app, accessed via in-feed teasers, a tab of the Explore page, promo stickers in Stories, and profile tabs.