I think what you're getting at here could basically be summed up as "They both give bonuses so they could be merged." I don't think it's that simple.
There is quite a bit of distinction on the function of their bonuses. Bonuses for religion were quite customizable, and often focus bonuses towards things directly on the map. The bonuses also incentivize the player to proselytize their religion to other factions (not just through the potential of a religious victory; there are direct bonuses for spreading your religion).
The bonuses for ideologies in Civ 5 generally focused on helping urge you towards a specific victory path. Moreover, the spreading of ideology was much more passive and indirect; the only real agency a faction has here is in obliquely defending themselves against other ideologies by generating as much Culture and Tourism as possible. The spreading of ideology was moreso a tool to punish other factions, rather than to help your own. Forcing someone's hand and making them switch ideology is devastating in Civ 5. You lose an entire turn to anarchy--no yields at all--and you also lose all of the other bonuses you had accumulated for your existing ideology. There's no such punishment for religion, really.
Ideology also divided the world into blocs, while religion doesn't. Even if the world's religions could be consolidated into a few on the map, only 1 faction is going to be fervent about defending and spreading it, and only 1 faction reaps the lion's share of the rewards. Ideologies are in contrast meant to be shared and are "unowned." This distinction, and accompanying AI diplomacy modifiers, really facilitates separating factions into blocs, which accelerates conflict and the end game.
On the one hand, you seem to be arguing that Religion and Ideology as modeled in Civ V are different, which may be so (it's been too many years since I even opened Civ V for me to argue that one way or the other) but is irrelevant here, where the question is whether they are inherently different and therefore must be modeled differently in Civ VII.
And I argue that it ain't necessarily so.
Religion did divide the world into 'blocs': the terms 'Christian Europe' and 'The Arab World' (meaning Moslem) were very real and used for centuries and were defining terms even though neither faction was politically homogenous for most of that time.
And neither Ideologies nor religions were necessarily willing or able to spread. In fact, one distinguisher among religions is between those that were evangelical and spread to numerous populations, like Christianity, Buddhism, and Islam, and those that were as much ethnically/natively based as religious and did not spread at all outside their original cultural/ethnic group: Shinto and Zoroastrianism for two prime examples.
Virtually all the Ideologies, regardless of how doctrinally committed to spreading, developed distinctly nationalistic and exclusionary versions of themselves: Russian Communism was Russian first and Communist second (witness the inability to convince either Tito's Yugoslavia or Mao's China to adopt it in precisely the same form and the abrupt collapse of the Warsaw Pact members once Russian arms were not guaranteeing their membership in the Russian Communist bloc), and 'Fascism' in Europe, at least, simply became a general term for ultra-conservative ultra-nationalism and little cooperation among them at all. To quote another historian, "The velvet glove of Nazi Socialism was too threadbare to hide the mailed fist of German Nationalism" - and that could be applied with less success (chiefly for lack of sufficient 'mailed fist') to every fascist government.
Please understand, I am not trying to argue that Religion and Ideology are identical except on the most ephemeral terms, simply that they share a number of similar characteristics IRL that could be usefully used in game versions of themselves.
Yes, Religions could have serious benefits from spreading to other Civs/Factions - but not all of them. Yes, Ideologies were and are represented as Monolithic Blocs, but that was never true and doesn't have to be modeled that way - although relations between different Ideologies (or Religions) should have a component of negative/antagonistic effects.
How much the game might be served by having a distinctly separate Ideology mechanic in Late Game is yet another question, I think. My own feeling is that the conflict between Ideological blocs could be equally and more realistically (for what that's worth) well modeled by emphasizing Nationalism, a distinctly exclusive 'ideology' and its effects on the more 'universal' Ideologies like Communism or Fascism, which in fact weren't universal as they were applied IRL. Rampant Nationalism, whether applied to Communist, Capitalist, or Fascist state mechanics, would accelerate conflict in the late game as much as a semi-fantasy monolithic set of Ideological blocs.
It could also, again as IRL, potentially accelerate some kind of Supra-National cooperative mechanism, like the League of Nations or United Nations (or a much, much, much better done 'World Congress'), which would also be a unique-to-the-late-game mechanic to keep things interesting.
