The issue with BMW is that they are a relatively small manufacturer and if the had dedicated theirselves to EVs and made a separate platform, that would lead to losing the economies of scale for their ICE cars and in result losing instead of making money with the ICE vehicles. So they bet on the ICE and f#cked the EVs instead of doing the opposite.
Why would any legacy carmaker bet on EVs
today when a vast majority of new cars is still ICEVs or at least PHEVs? They do follow the deadlines set by local (namely USA, EU, China) legislatives regarding emissions - and thus transitioning from ICEVs to BEVs properly according to those deadlines. Every legacy carmaker has the same goal, and the same plan. And the most deadlines are set in stone.
When it comes specifically to BMW ... They opted for all-purpose platform for the transition period, so they can best optimize the production - being flexible to respond to demand: being that ICEVs, PHEVs or BEVs. Mind not all markets around the globe are at the same point of transition. Some are well ahead, some are well behind. So flexibility is the key here. Bigger carmakers (eg. VAG) opted for separate platforms & separate production lines, even separate plants. BMW are not in a position to be able to pull something like that. BMW AG is just too small for something like that.
So, in this transition period BMW will offer all-purpose platform and different drivetrains of same models. Therefore iX3, i4, i5, i7, iX1, i2 etc.
After that (after 2025) BEV-specific platform for high-volume models will emerge, while some non-BEV (PHEVs) versions will continue to utilize the all-purpose platform a bit longer than usually.
As you said: perhaps development of all-purpose platform is more costly & full of compromises, it allows a much more flexible (and cheaper!) production & response to the demand of different type of powertrains.
BMW have made quite some calculations, and decided it's better for the to transition to BEVs slowly - and when they go mass-volume, they can offer the latest tech. Also their ICEVs still make more money then BEVs (even with all the emission fines they are paying).
So, BMW can't afford to make mass-volume BEVs & losing money on them. That's not the option. Therefore they are slow. Also with new battery & charging tech, AI, 5G / 6G. IoT emerging coming to the market slowly is not the worst decision ever. Sure they can't compete with BEV-only manufacturers like Tesla, but they are (still) very much on par with other legacy carmakers. And don't worry ... when time is due ... when BEVs will be present in mass volume BMW & other carmakers will be ready.
What (still) worries me more is how legacy carmakers will handle the IT, digitialization ... and mostly the content & services connected to that. I'm not sure they will be able to compete with big IT giants when it comes to that. Just like Android smart phone makers can't. Mind that in future services / content will contribute at least 60% of revenues in the mobility sector. Hardware makers (= car makers) will be left with less profitable products - automobiles, while content / services provider will thrive more. It's inevitable.
That's the real threat to automotive industry. Not the electrification of drivetrains. Everyone will go BEV & offer mass-volume BEVs in the near future. But services & content ... That's the field car makers can't really compete in & offer competitive products (content, services) to the products IT giants do offer.