Dota's weird where heroes and cosmetics fluctuate based on meta, people tend to specialize to 2 roles at max so over time peoples interests shift. Not only that but exclusives give permanent benefit to existing players but penalize all future players for the rest of the games lifecycle – seems absurd any developer would think this model was a good idea.
Solution 1: Have 2 year timed exclusives. The BP version can be unmarketable but offer newcomers a marketable version. After 2 years nobody gives a shit about anything and the BP owners still get their exclusive fancies. Also if a hero has changed drastically or you have discovered a new hero you aren't permanently excluded from getting their cosmetics.
Solution 2: Offer all cosmetics for temporary rental using Dota+ Shards. Didn't get the IO Arcana and Valve don't want to upset the crybaby 'muh exclusive crowd'? Sucks, but you can unlock a temporary version with your D+ subscription that expires. Arcana progress is still tracked if you lose access, but you gotta keep paying shards if you want the goods.
A few more reasons to why I don't think exclusives should be permanent;
Punishes people for their temporary life circumstances i.e. if a person doesn't have an income that allows them leisure expenditure or couldn't play Dota or just plain didn't know the game existed; they get permanently locked out of the system. New player that discovered a love of Earthshaker? Lol you didn't play Dota in <INSERT BP DATE HERE>? you get NOTHING, you lose. Good day sir.
There can only be 1 of each hero in a game; So having in game comparisons of cosmetics isn't possible. People just like the appeal of 'perceived rarity' for example if you are using the Windranger Arcana – you know the money/effort it took to grind and thus place a higher 'perceived rarity' on the item. You can play 10 games and encounter someone else with the Arcana and it doesn't detract from your perception of how rare it is as you only recall your personal experience in obtaining it – so having items be exclusive is counterintuitive as personal perception of rarity is seemingly stronger than actual rarity. To get to the point I am saying that making an item exclusive is not what makes people think an item is rare – rather the difficulty in obtaining it – so locking future people out of means of obtaining is unnecessary. Also people care more about their own image, we all appreciate other's cosmetics too, but most just care what they have for themselves – it was never a desire of any of us to lock others out of obtaining cool stuff.
Heroes/metas shift. I gave QoP as an example in the headline as being the most recent hero to get a playstyle shift going from pos2 to being a pos5. This has happened with other heroes too; Io was probably the first hero that was perceived as a Support for a very long time in Dota 2 and afterwards shifted heavily to a core role. Core players who would never touch the hero suddenly got penalized for circumstances few could have predicted at the time. It makes me wonder as a support player should I strive to get the Spectre Arcana, I mean who knows what the hero would look like in 2 years. Maybe Spectre could become an offlaner tank that initiates – stranger things have happened.