This is an 'Outstanding' product. You can tell it's been thoroughly well thought out.
I've tested the vast majority of membership plugins, PMPro, PMS, ARMember, Sure Members, Ultimate Member, S2, MemberPress, Wishlist, WP User Reg, Memberful. ProfilePress (PP) is definitely in the top 3 plugins I've tested. It's the small things that you don't think about that make the real difference.

Here are just a few of the minor (but hugely impactful) differences.
1) It has Passwordless login. But it's the best execution i've tested. Some other plugins offer this, but they send the customer an email with a long URL, however PP sends a clean email with just a simple button.
2) Be aware of stripe and scripts (or whatever the correct technical term is) adding weight and slowing down your website. PP seems to only load scripts on the relevant pages and not all pages (I'm not technical so forgive my lack of knowledge).
3) The checkout layout is awesome, it's modern and compact, and you can customise fields. Some other plugin checkout screens are very longwinded. You can even remove username so customers can just use emails to login (some other plugins do also allow this, but it's very important feature to have).

There are lots of other reasons, but I just wanted to highlight a couple of minor (but important) ones that you may not have considered.

While I’m here, can I make a few suggestions about future improvements please?

This is (hopefully an easy) quick win:
1) Please could you add 'Affiliate Coupons' with SliceWP. So affiliates can just give a code discount code to customers and still generate a commission without having to use the referral URL. PMS and a few other plugins have this function, here is a link to PMS on SliceWP.
https://slicewp.com/docs/integration-paid-member-subscriptions/
To make this feature on PP even better than PMS, would it be possible to have button on the customer dashboard that will 'auto generate an affiliate discount code' that they can use straight away, rather than me having to manually create an additional discount code in PP for each person.

These probably require a bit more development work but would be vitally important:
1) Gift purchases. Many other plugins have a gift purchase function, however most of them require purchasing a coupon code which is then redeemed. It is better to be able to purchase a plan straight away and gift it to somebody. This is how PMS does it which is excellent. It also allows the gifter to personalise a message to the recipient. Could you copy this?
www.cozmoslabs.com/docs/paid-member-subscriptions/add-ons/gift-subscriptions/
To make this feature on PP even better than PMS, would it be possible to set a send date on the personalised message so that the recipient can receive it on a specific date and time, for example their birthday.

2) Group memberships. Several other plugins have this function. The two are the best providers of this feature or PMS and PMPro
www.cozmoslabs.com/docs/paid-member-subscriptions/add-ons/group-memberships/
www.paidmembershipspro.com/add-ons/group-accounts/
This would also be useful for customers to purchase a Family Package to share with family.

3) Membership Cards. Several other plugins provide digital membership cards with a QR code and Photo, which customers can download and store in Google or Apple wallet. If you could copy this this would be brilliant.
I WOULD PAY YOU EXTRA FOR THIS FEATURE
Also, if you could add a feature that allows the QR code to be scanned on an attendance register so we can see how many times a member has attended an event.

These are minor improvements:
1) On the checkout screen the 'Have a coupon?' is a little bit too hidden and hard for customers to spot. Is it possible to make it a little bit more visible. If we could choose to unhide it by default in the checkout display settings, or change it’s size please.

2) On the checkout screen, where it says 'Already have an account?', I cant seem to modify it's description. Reason is because I like to just offer email as the login, and remove all references to 'username'. Also it does not have a description about using Passwordless login, even though it does work.

3) Passwordless login. Is it possible to have password and passwordless login on the same login box? At the moment you have to have two separate login screens because if the password field is showing then it is 'mandatory' to enter into it. PMPro has a really good solution to this by having a 'Request email login link' button on the same login box.
www.paidmembershipspro.com/add-ons/magic-login/

3) Save membership cancellation and recovery. If a customer wants to cancel their subscription, could we offer them some additional discounts to tempt them to reconsider. ARMember has this feature.
www.armemberplugin.com/addon/membership-retention-addon/

I would be happy to be a test customer for your development on any of these features.
Thanks