Are marksmanship badges just like diamonds and are forever?

I think that while you are on active duty you are required to requalify semi-annually,so you should wear the badge of your most recent qualification. So skill do deteriorate if not kept up. You may have qualified Expert in BCT with a Drill Sgt standing over you telling you what to do. Then ten years later you can't hit the broad side of a barn. I have served as a range safety for units where a senior SP4 didn't know how to insert a magazine in a weapon.

The one and only clasp I have heard of that was permanent, was grenade. I do not know if that is truth or not. I earned mine in basic, and never threw again. So, I really can not say. But, what I do know is, that I did swap badges between Expert and sharpshooter many times while I was in the Army. Although, I do carry the expert rifle ribbon from Air Force Basic and that was back in 1984.

In the Marine Corps you must qualify every year or have an explanation as to why.

No they do change each time you qualify, thankfully mine all stayed the same, looks like the way they're arranged at the top. Mine goes expert- rifle, sharpshooter -pistol, marksman- grenade.

They're temporary, if you shoot expert with an M4, you wear Expert - Carbine. 366 days later you shoot Marksman with an M16, you take off the Expert - Carbine and put on Marksman - Rifle, 2 days later you actually go to a grenade range and do the grenade assault course as a Sharpshooter, you add that as Sharpshooter - Grenade, to the right of your Marksman - Rifle.
If you go to a qualifying competition and get the badge - you will be permanently awarded the Distinguished Badge for that weapon system.

The regulation states that marksmanship badges are revoked "whenever an individual, upon completion of firing a record course for which the previous award was made, has not attained the same qualification". That's the only cause listed for revocation of the badge. It does not say anything about expiration, time lapses or any other cause.

There is a seemingly minor misquotation at work here, with considerable impact on the meaning of the reg.
You quoted the reg at 8–47 as "an appropriate bar is furnished to denote each weapon with which he or she qualified". You later referenced 8-47 as stating "to denote each weapon with which he or she is qualified."
Assuming your original quote of the reg at 8-47 is accurate, it refers to an historical, documented fact -- the individual is awarded a bar for a "weapon with which he or she qualified". In other words, they have indeed both fired the course, and qualified at some skill level meriting an award.
Your later reference to 8-47 changes that. The change to "is qualified" connotes currency in the skill level. That was not present in the original text of the reg at 8-47 --- only the fact of having fired the course and having qualified at some undetermined time in the past is stated as pertinent.
It appears to me that a strict reading of the reg would imply that whatever qualification, with whatever weapon, remains in effect, unless and until the individual again fires that weapon for qualification.
Of course, it seems foolish to an old Marine like me to see "appropriate bars" for expert qualification for such weapons as e.g. "hand grenade". I begin to wonder if there is a V-ring in the hand grenade target. But if the Army issues that bar for that weapon, it looks like it remains in effect until a re-qualification takes place.
I recall that I fired a 224 in boot camp with the M-1, and only made sharpshooter. My reserve unit could not schedule a re-qualification firing, so my unit spent a half-day firing a "familiarization course" instead.
Should I have removed my sharpshooter badge, because it was a little over 2 years before I could re-qualify (as an expert rifleman)?

Always an exception. The Excellent in Competion badge is a permanent award. Orders are cut for this award. Can be awarded for rifle or pistol and takes precedent over qualification awards. Covered by AR 350-66.

I did a records check a few years ago thinking they'd (st Louis) just send me a listing. They didn't. They sent every medal, ribbon, appurtenances and badges. Yes it included expert rifle and grenade bars.

Must be a 2Lt masquerading as a seasoned officer.