In the beginning, J2ME has pretty good vision, write once run anywhere. However, although J2Me has standards API, in the reality, almost every developers in the J2ME worlds do: write once debug anywhere. The problems in J2ME is application porting. Except you are a developer with deep pocket (can test on almost every J2me devices), the proposition of WORA is a bit flaw (How you can get the return on investment if you need to invest on lots of test devices).
Second, the market of mobile java (especially gaming) is regulated by the network operators, almost 90% of people buy their game through mobile deck. The operators only deal with very limited number of developers company. No matter, how innovative or good your java apps, if you do not have channel to network operators, you could not make money.
The situation will get worse for small developer with the introduction of Java Verified program (Write Once Pay Everything). This program is the minimum requirement to apply for Java apps publication on manufacturer portal and network operator.
How can the developers justify to spend between (200 Eur - 500 Eur) to certify for this program, if the average sales on almost 80% of java apps on the market today are between 0 USD up to 25 USD (just look at the Handango portal of Java apps, you will have rough picture of the java apps sales figures).
About the future of Symbian, I do not really know. Since Symbian apps are distributed through portals. I think, there are no significant markets for Symbian apps. How many company makes really money with Symbian apps? I guess that 80% of developers/companies who sell Symbian apps do not really make money.
I think the real threat/challenging is not Microsoft. Microsoft can not enter the mobile market, because its proprietary and close technologies will not be accepted by network operators who control the gateway in this mobile industry (networks operators do not want to be commodity services, such as in the PC hardware industry, if the Microsoft enter the market).
I believe that real challenge is the network operators who act as gateway between the market/consumers and the developers. Currently, sad enough, Network companies who decide which apps are good and not the consumers. This condition will play a major role for the future for mobile developers.
Could someone comment on this or share his/her experience?