You're going to end up with other problems with IBM microdrives. At that point, why not just use a regular SCSI hard disk?
That said, the Pi (especially the original one) is going to live on swap if you have one of the older ones with 256 megabytes of RAM and you're using it with Linux that has a graphical interface. 256 megs just isn't enough for most Linux setups with GUIs, and if I'm honest, it's barely enough for Ye Olde Text Only Install of Debian at this point.
The thing that really kills flash storage is the write cycles. For all intents and purposes, you can data on a flash media, and read to it an infinite number of times, until the heat death of the universe, and it'll never be corrupted.
So, for the purposes of 68k Macs and early PowerPC systems, you can happily replace the spinning hard disk with either a good SD card or a good CF card. The one thing I recommend considering is trying to run the system without virtual memory. I absolutely and categorically recommend against turning off swap on Mac OS X, literally any UNIX/Linux system, and any Windows NT system. (And it's probably a bad idea on Windows 9x but I'm less familiar with that) -- however, for the most part, system 7 Macs were so slow that whatever difference it'll even make on System 7 (if there is any) is going to be nearly negligible.
Mac OS 9 is kind of a toss-up. I've heard informally that Mac OS 9 is sped up with VM on (even if you have like a gig of RAM) because it allows you to not actually read the entire app file from disk when the app is launched. But, if you're using a faster disk with a lower seek time and there's no limits on reading, then you may not need whatever speed boost would have come from enabling/allowing VM to run.
On Mac OS X, Windows NT, and other Linux/UNIX systems, your computer will crash and burn without enough memory, and with virtual memory or swap turned off. In many cases, there'll be no warning and it will be exceedingly ugly. I can't possibly say enough that I recommend you just don't.
That said, if your machine is also networked to a generously big netatalk server, you can copy the contents of your hard disk to a Disk Copy image stored on the network and then use the 7.5 Network Access Disk should you ever need to restore. (Just be sure the Disk Copy application itself is on the file server, or the 7.5NAD.)
Also, take care of what media you're buying, Samsung Evo media may be worth looking into, and if you're really worried, go for the AztecMonster (or another CF adapter) and a particularly high end card that's meant to endure a large number of write cycles.
For newer PPC PowerBooks with IDE in them, you can consider a regular IDE SSD or an adapter to use an mSATA/m.2 SSD. Modern SATA/M.2 SSDs have completely insane write durability. Most of them are rated to be able to fill the disk over and over again at full speed for 50 years, and many of them will actually last longer than that.
For PCI Mac desktops, you can buy a SATA card and install whatever hard disk or SSD you want.
Another overall option for SCSI is to buy used server hard disks. I can only really recommend this for the biggest and best Macs that have a lot of cooling. I fear the relatively small space and low airflow of something like a Mac IIsi or an LC series system would cause the drive to ultimately die of heat exhaustion, but some people have been running theirs for a while, I don't know under what circumstances.
You'll probably end up paying something for either solution, and for me, I'd rather go with scsi2sd or aztecmonster, knowing that the media's going to be pretty good, and the adapter itself will be relevant for the foreseeable future.
