Essentially, regarding irrealis conditionals, it is French that has changed "further" than in English in this regard.
In Classical Latin, both the protasis (the if-bit) and the apodosis (the consequence) would both be subjunctive, either imperfect or pluperfect depending on tense.
Nam nisi multorum praeceptis multisque litteris mihi ab adulescentia suasissem, ... numquam me pro salute vestra in tot ac tantas dimicationes atque in hos profligatorum hominum cotidianos impetus obiecissem. (Cicero, Pro Archia)
For unless I had persuaded myself from my youth with the teachings of many, and by many books, ... then I should never, for the sake of your safety, have subjected myself to so many and such great struggles and to the daily attacks of desperate men.
But it seems Vulgar Latin / Common Romance went with what was the pluperfect subjunctive and had it replace the former imperfect subjunctive, and named it... the imperfect subjunctive. They then (mostly) dropped the subjunctive from the apodosis and replaced it with a new form, cobbled together from the infinitive with the imperfect indicative of (whatever became of) habeo ("have to", in its extended meaning of "must"), which became known as the "conditional" mood. [Note: Standard Italian ended up using the derivative of the Latin perfect indicative, which became the Italian passato remoto, usually called the "preterite" or "past historic". Old Tuscan did use the imperfect indicative of avere though.]
New compound tenses were based on this paradigm. Hence, modern Spanish (with pluperfect subjunctive and conditional perfect):
Si no lo hubiésemos explicado antes de la foto/ , no habría resultado sencillo saber qué imagen se correspondía con algo bueno para Neymar. (El País)
If we had not explained it before the photo, it would not have been easy to know which image corresponded with something good for Neymar.
Old French also did this:
Car la tenisse en France et Bertrans si i fusset, / A pis ed a martels sereit aconseüde. (Pélerinage)
As if I had kept it in France, and if Bertrand were there / With pikes and hammers (she) would have been attacked.
But this coexisted with the same construction as Classical Latin (subjunctive in both clauses):
Se je ne fusse en tel prison, bien achevaisse cest afere. (Le Vair Palefroi)
If I were not in such a prison, I would put an end to this matter.
... and with what Modern French would do, which would be with imperfect indicative with the conditional present.
Et se je pooie amander la mort don je n'ai rien mesfet, je l'amanderoie sanz plet. (Yvain)
And if I could pay death which I have never wronged, I would pay her without complaint.
By 11th century, the conditional tense was replacing the subjunctive in the apodosis, but the indicative replacing the subjunctive in the protasis took longer, persisting into early Classical French of the 17th century. One idea is that it naturally emerged in Late Latin where the Latin imperfect subjunctive was disappearing and hence they needed something for this present counterfactual construction, with the fact that it was more "present" being allocated to the imperfect indicative.
However, as one forum discussion explains, the disappearance of the imperfect subjunctive from the protasis is the major thing that needs explaining. One article from 1901 suggests that the imperfect subjunctive fell out of favour because the endings -asse/-asses/-assent (all pronounced /as/ in IPA) ended up sounding "coarse" because of its relation to the derivational suffix -asse (those who know their French swearwords will testify to this). Interesting theory, but not necessarily satisfactory.
What happened then to the past subjunctive in English? It seems to be me that it was swept up in the general reduction of endings, and then the replacing of constructions using the past subjunctive conjugation with auxiliary verbs like should and would. The bulk of this change happens in Middle English. But the past subjunctive persists very clearly in Middle English, as Middle French translations from the period attest.
However, Old English does not seem to use the subjunctive forms in conditionals, and it is Middle English that appears to innovate this (in both factual and counterfactual), with past subjunctive in both protasis and apodosis in counterfactual conditions, although normally the apodosis uses a modal auxiliary in past subjunctive (e.g. sholde). Old English does use subjunctive forms a lot more as a whole though.
and I were a pope/ Not oonly thou, but every myghty man,/ … Sholde have a wyf;
If I were a pope, not only you but every mighty man would have a wife.
Another factor that seems to keep the subjunctive in Middle English is the use of inversion to signal the protasis, instead of explicit conjunction if, unless. This persists into modern English, in relatively set phrases:
Were he still here, ...
In conditional sentences, there is a moribund distinction drawn between indicative and subjunctive in English:
If he/she was singing, you would have certainly appreciated it.
If he/she were singing, you would have certainly appreciated it.
... relating to counterfactuality.
Is there any meaningful comparison to be drawn here? It doesn't seem like it to me - there is a completely different set of constructions that the two languages have charted to produce counterfactual conditional statements.