Instead of removal, repair a molar with cusp broken off, below gum line, to keep it healthy for a decade. Any suggestions?

Impossible to give you a definitive answer without a photo and xray of the tooth.  Your options will likely be to crown the tooth unless the tooth is  broken too far under the gums.  If this is the case, you will need to consider extracting and placing an implant as your dentist explained.  My guess is the implant would be the better long term solution, but can't say for sure without the proper information to look at.  Good luck,  

With today's advanced materials and excellent techniques it is definitely possible to save a molar that has a fracture below the gum.  You will require gum surgery and a minimally invasive porcelain onlay to rebuild the tooth, depending on the depth of the fracture and the status of the nerve you may need a root canal as well.  Good on you for exploring the option of saving the tooth rather than yanking it out:)Best of luck.

   Again, an X-ray will help, but wait a minute!! How about crown lengthening by your neighborhood periodontist of which there are several around Seaside.  That way, with an endo & crown you get to keep the tooth for at least several years!

If you have enough bone around the roots of your molar, your tooth can be saved!!!   Because it broke below the gumline, it probably will require  a crown lengthening procedure.  Depending on how far below the gum that it broke and how close to the level of the bone supporting your molar will determine the type of crown lengthening.  It it is at least 2 mm above the bone level(x rays will allow this to be determined), then only a soft tissue crown lengthening will be required using either a laser, scalpel, or electrosurgical unit to remove some of the soft tissue covering your tooth down to the break.  If there is not 2mm of bone, then you will require an osseous crown lengthening procedure where some bone is removed from around your tooth to create an area of 2mm of healthy tooth structure above the bone level.  This will allow your gum tissue to heal and be healthy.  You may also require root canal therapy, depending on severity of breakage and your symptoms now and after crown lengthening and preparation of your tooth for a new crown.  If you have pain or extreme sensitivity, you should consider having the root canal prior to final cementation of the new crown.  The costs for these procedures can add up, so your other alternative is to have the tooth removed and have an implant and crown to replace it.  If you really want to keep your natural tooth, and expense is not a concern, then the above described procedures should make your desires possible.  It is also important that your entire mouth is free of gum disease and that you visit your dentist regularly for maintenance, and that you practice good oral hygiene at home.  Anything otherwise, and these treatment will not last a long time.  Good luck and let us know if you have any further questions and how you make out.