An interesting development happens to Indian history after the Magadhi era : while ALL subsequent native empires of India aspired to follow the Magadh's conquest of ALL of jambhudwip (Indian subcontinent), they ALL resorted to the pre- Mahapadma period of vassalisation model, instead of annexation model.
The cause of this shift is speculative, but an explanation that makes sense to me is as follows:
The most conventional argument against vassalising ruling dynasties is that when your own dynasty becomes weak or undergoes succession crisis ( which includes a juvenile king under guardianship), the vassalised dynasties tend to break away and you have to wage war again to bring them under your control.
Indeed, this may've served as the prime motivation of Mahapadma to annex the whole goddamn ganges basin.
Annexed territories, on paper, tend not to break away under the slightest succession problems or uncertainty.
But this only works, when the annexed territories have cultural & linguistic conformity and are not composed of heterogenous culture zones.
Plus, as the experience of the post Ashokan Mauryan dynasty showed, annexed territories' governors are also prone to breaking away if central rulership is weak.
However, vassalised territories offer one benefit over annexed territories and that is,more efficient local rulership: the centre isn't directly in control of appointing & dismissing & keeping an eye on distant local rulership, that task is delegated to the local dynasty - who if incompetent, simply has their ruler replaced ( most likely by their next generation). The feudal nature of this period also makes the locals less likely to rebel, as their rulership is done by their own local administration, rather than a governor appointed from distant lands.
This is why, in my opinion, all subsequent Indian dynasties show annexation tendencies over a culturally homogenous region - such as the Guptas, Pratiharas, Pals, Chandelas etc ( who all try to/succeed in annexing the indo-gangetic basin) or even in the south, where the Karnata dynasties such as Chalukyas, Rashtrakutas, etc annex the Kannada territories only, while establishing vassalhood over lands that have a differing culture from them.
Indeed, to my knowledge, in the entire post-Magadhi period, the only notable 'culturally different' area directly annexed into the core territory of an empire is by Chandragupta Vikramaditya, who annexed Gujarat into his empire after the destruction of the Western kshatrapa dynasty.
Just my two cents.
