No, it doesn't. A better package manager doesn't magically fix deficiencies in the packaging (noticing ABI changes), nor design intricacies (r/o rootfs + overlay), nor immutable packages (e.g. kernel (OEM bootloader limitations) & libc & busybox & ...), nor does it magically retrofit 'enough' flash space for soname concurrency, nor.... It's just one piece in the puzzle, among many others.
To quite some extent these deficiencies are what makes OpenWrt possible on heavily ressource constricted hardware with very limited partitioning- and bootloader options in the first place. Just compare the minimum system requirements of OpenWrt vs Alpine vs Debian, Fedora, ...
EDIT: realistically, if you really want in-place upgrades comparable to e.g. Debian, your lowest common denominator for all supported devices would need to be:
- at least 5 times the minimum (flash-) storage requirements (and that's very much on the low side, in practice I don't think you'd succeed below ~1 GB usable space across all supported devices)
- a recoverable bootloader, so a means to toggle between at least two different kernel versions AND an ability for some kind of -easy- disaster recovery (in the sense of being able to boot from removable media or reflashing over serial or USB)
- a way for some flexibility in the storage partitioning, so no hardcoded assumptions (basically means eMMC, sdhc, SSD, HDD storage and a smarter bootloader than we actually have on most devices)
If you want OpenWrt to change into this direction, literally all supported devices would need to meet this as a hard minimum - right now, apart from x86_64 and the ARM SBCs (rockchip, RPi, sunxi) zero of the currently supported devices would meet these minimum requirements. Filogic might (provided they come with >=1 GB storage), with some changes on the bootloader side (they have USB based recovery) - apart from that, …crickets.
Having to cope with tiny main storage (>=8 MB), small RAM and ancient and very limited bootloaders with very limited interaction possibilities and bad recovery means just require making hard choices, which have consequences on 'ease of use' (and in-place upgrades just need to be sacrificed to allow a few dozen MB of flash).
It's easier to make (e.g.) Debian a decent router (on x86_64 or similar) - than to run Debian on the hardware OpenWrt typically deals with (actually impossible for >98% of all currently supported devices).
If you wanted to make OpenWrt fully support partial in-place upgrade, it would inevitably lose the ability to run on (almost all of-) the devices it currently supports - at which point it would just be another linux distribution without any distinguishing advantages (apart from a pretty GUI, which is the easiest thing in comparison, so 'just' start out with Debian and write a GUI).